# Troubleshooting softphones in Citrix and VMware virtual desktops

Canonical: https://warmtransfer.net/knowledge/vdi-softphone-troubleshooting

Last verified: 2026-10-02

Microsoft Teams, Webex App, Zoom and Amazon Connect each document a mode in which softphone media is not optimized for the virtual desktop, and Cisco says that without VDI optimization, Webex App calling on a hosted virtual desktop is not supported because media would hairpin through the data center[^51][^40][^59][^17]. Troubleshooting starts by establishing which mode a user is actually in, using each vendor's own status strings, logs and process checks[^41][^52][^13].

## How optimization and fallback work

When Teams SlimCore-based VDI optimization fails to initialise, users can end up in a fallback mode reported as "SlimCore Media Not Connected" (server-side rendering), which is distinct from being optimized on the legacy WebRTC stack[^40]. Webex App VDI fallback mode gives short-term support for basic audio and video calls when the virtual channel cannot be established, at lower call quality and without the full feature set[^50]. In Zoom VDI fallback mode, all media is sent from the remote device to the virtual desktop instead of directly to meeting participants[^59].

In one common Amazon Connect VDI model, a custom CCP with no media runs on the virtual desktop for signalling and call control, while a standard CCP on the agent's local desktop handles media[^24]. When the VDI client has no local browser, Amazon Connect describes a single CCP running media on the VDI server, usually with UDP audio enabled in the VDI OS, and says this model needs extensive testing to tune VDI server parameters[^23]. For Citrix VDI, Amazon Connect audio optimization uses a custom CCP built with the Connect RTC JavaScript library, which integrates with the Citrix Unified Communications SDK (UCSDK) to redirect media to the local desktop[^12].

WarmTransfer's reading of the sources is that for Teams, Webex App, Zoom and Amazon Connect, the fallback or unoptimized mode puts media processing back on the virtual desktop, so "optimized but degraded" and "fallback" need different evidence: the vendor's own status indicator, not call quality alone[^58].

## Diagnosing Microsoft Teams

### Read the status string

If a Teams VDI user sees "Azure Virtual Desktop Media Optimized" or "Citrix HDX Optimized" instead of SlimCore, they are on the legacy WebRTC stack, and errors 2000 (No Plugin) and 2003 (Virtual Channel not allowed) are the most likely causes[^41]. When Teams shows "Azure Virtual Desktop SlimCore Media Not Connected" or "Citrix SlimCore Media Not Connected", Microsoft names MSIX or AppX deployment errors as the most likely cause[^43]. The first time Teams detects the endpoint plugin, it needs 1 restart to move from WebRTC to SlimCore optimization[^46].

### Check the endpoint plugin and virtual channels

- On Citrix, the Virtual Channel Allow List policy in Citrix Studio must allow the custom virtual channels MSTEAMS, MSTEAM1 and MSTEAM2; if they are blocked, Teams logs error 2003 (deployErrc 16026)[^28].
- On a Citrix endpoint, Microsoft says to use Process Explorer to confirm that the wfica32.exe process has loaded MsTeamsPluginCitrix.dll[^27].
- On an Azure Virtual Desktop endpoint, Microsoft says to confirm that the msrdc.exe process has loaded MsTeamsPluginAvd.dll[^26].
- Microsoft's AVD troubleshooting page says Teams media optimization is supported only with the Remote Desktop client on Windows 10 or later or macOS 10.14 or later, and not for Teams running as a RemoteApp on macOS endpoints[^1].
- The Remote Desktop WebRTC Redirector Service is not pre-installed on the Windows 11 Enterprise multi-session + Microsoft 365 Apps marketplace images 23H2 (build 22631.3593) and 22H2 (build 22621.3593); the fix is to redeploy from a newer image or install the service separately[^2].

### Collect and read the logs

Teams VDI logs are collected by pressing Ctrl+Alt+Shift+1 in Teams on the VM, and Vdi_debug.txt inside the resulting PROD-WebLogs zip is the main file for VDI information[^42]. In Vdi_debug.txt, connectedStack "local" means the virtual channel connection failed and the user is in fallback mode, while "remote" means Teams reached the endpoint over the virtual channel but does not guarantee the calling stack initialised[^29]. Vdi_debug.txt also records version-mismatch evidence: bridgeVersion (tied to the Teams client on the VM), remoteSlimcoreVersion (SlimCore on the endpoint), pluginVersion (plugin in the remote desktop client) and rdClientVersion[^45].

Teams VDI connect and disconnect events are logged in the VM's Windows Logs\Application under source "Microsoft Teams VDI", Event ID 0, and filtering by that source first requires registering it with New-EventLog from an elevated PowerShell[^39]. The Teams client health page in the Teams admin center lists "VDI optimization failure" errors with descriptions and remediation actions on its Issues tab[^47].

### Interpret the error codes

A deploy error (deployErrc) happens while the plugin downloads the SlimCore MSIX from Microsoft's CDN and stages it through the App Readiness Service, and a load error (loadErrc) happens while starting MsTeamsVdi.exe and opening an RPC to it[^30].

- 2004 (deployErrc 16032): the Citrix Workspace app version on the endpoint is not supported and must be upgraded[^34].
- 2005 (16043): Teams is running as a Citrix published app or an AVD/Windows 365 RemoteApp, so it does not load SlimCore and the user is always optimized with WebRTC[^35].
- 403 (deployErrc 3227): something on the network, typically a proxy, is blocking the SlimCore MSIX download; 404 (3235) points to a missing CDN package or proxy interference[^37].
- 1260 (deployErrc 10083, Event Viewer 0x800704EC): Windows Package Manager usually cannot install the SlimCore MSIX, often because of AppLocker, and the fix is to disable AppLocker or add an exception for SlimCoreVdi packages[^31].
- 16389 (E_FAIL): Microsoft names the BlockNonAdminUserInstall policy, including the Intune "Block non admin user install" setting, as the most likely cause, and when it applies SlimCore registration aborts and the call falls back to non-optimized or, where available, WebRTC[^32].
- 16389 also has 2 causes that Microsoft lists as fixed in plugin version 2026.29.1.4: SlimCore Host/Framework registration mismatch and the shared-kiosk multi-user registration issue[^44].
- 4390 (ERROR_NOT_A_REPARSE_POINT): a SlimCore MSIX error seen on thin clients that use Unified Write Filters and RAM-disk overlays[^38].
- 1722 (RPC server unavailable): commonly occurs on slow thin clients overloaded at logon or with delayed AppX registration, and Microsoft says to treat it as transient if optimization eventually succeeds[^33].
- 3000 (deployErrc 24002, SlimCore deployment not needed) and 3001 (24010, SlimCore already loaded): not errors, because they show the user is on the SlimCore architecture[^36].

## Diagnosing Webex App

For Webex App VDI on Citrix with a virtual channel allow list policy, the Webex virtual channel CSCOTM must be added with the process path C:\Program Files\Cisco Spark\dependencies\teamshvdagent.exe[^49]. A correctly installed and working thin-client plugin makes the app's Health Check show a connected status for the Virtual Channel[^52]. On Linux thin clients, the Webex Meetings VDI plugin must be installed alongside the Webex App VDI plugin, while Windows and macOS use a bundled installer[^53].

New Webex App VDI versions are released every 2 months and stay backward compatible with earlier thin-client VDI plugins[^55]. In 2026, Webex App VDI slow channel releases are 46.3 (March), 46.6 (June) and 46.10 (October), with no April (46.4) release[^48]. Starting with the October 46.10 release, Webex App VDI ends support for Windows versions earlier than Windows 10 1809 and macOS versions earlier than macOS 13 Ventura[^54].

## Diagnosing Zoom

The Zoom VDI plugin on the thin client must always be the same version as, or lower than, the Zoom VDI client on the virtual desktop[^60]. A user in fallback mode sends all media from the remote device to the virtual desktop instead of directly to meeting participants[^59].

## Diagnosing Amazon Connect

### Requirements on every platform

Optimized Amazon Connect VDI audio needs the VDI server to reach Amazon Connect on TCP 443 and the agent's thin client or local device to reach Amazon Connect softphone media on UDP 3478[^25]. Amazon Connect VDI audio optimization does not support the native CCP, and with the native CCP, media keeps flowing through the browser inside the VM[^17]. Amazon Connect does not support Voice Focus noise suppression in VDI environments because it is incompatible with audio redirection[^18].

See also [Firewall ports and IP ranges for cloud calling](https://warmtransfer.net/knowledge/firewall-ports-uc).

### Citrix

The Citrix UCSDK is not enabled for Amazon Connect by default: the admin must add a ProcessWhitelist REG_MULTI_SZ value (Chrome.exe, msedge.exe) under HKLM\SOFTWARE\WOW6432Node\Citrix\WebSocketService on the VDA and restart CitrixHdxWebSocketService[^14]. To fix dead air and dropped calls during mute in older Citrix Workspace app versions, AWS says to set VDIPlatform to CITRIX_413 instead of CITRIX, use CWA 2508.10 or later on Windows, Linux and Mac, and use 2507 or later on ChromeOS, with CWA 2305 as the absolute minimum[^11]. AWS warns that Local Network Access prompts introduced in Chrome 147 and Edge 147 can block Citrix UCSDK optimization, so Amazon Connect audio optimization does not work until the workaround in Citrix article CTX696569 is applied[^10]. To confirm that audio is redirected, AWS says to check Task Manager on the agent's thin client for the running HDX service[^13].

### Omnissa

Amazon Connect audio optimization with Omnissa needs Horizon Client and agent 8.15.0 (Horizon 2503) or later and, for now, supports only Windows agent workstations, with 2503 being the first version to support ICE restart[^19]. When a CCP sets VDIPlatform to "OMNISSA", it does not fall back to standard browser audio if Omnissa optimization fails, so calls fail when an agent opens that CCP outside the Omnissa VM[^20]. With Omnissa, the Amazon Connect Salesforce CTI Adapter does not support VDI platform detection, so its media goes through the VM's browser rather than being processed on the client[^22]. To verify optimization, AWS says to check that the Horizon WebRTC browser extension icon is blue (Ready), that html5server.exe and wsnm.exe stay running during calls, and that the browser console logs "RTC.js is using OmnissaVDIStrategy"[^21].

### Azure Virtual Desktop and Windows 365

When VDIPlatform is set to AZURE, the CCP uses Azure (MMR) audio optimization only and does not fall back to browser audio, and if VDIPlatform is unset, Connect auto-detects and enables Azure optimization when available[^8]. The MMR extension redirects calls only for domains on its AllowedCallRedirectionSites allowlist; Connect-hosted CCPs are on the default list, but a custom CCP on the customer's own domain must be added by policy[^15]. To verify MMR call redirection, AWS says navigator.mediaDevices.isCallRedirectionEnabled should return true, and a call should still work with microphone permission disabled for the CCP domain in the virtual desktop browser[^9]. Amazon Connect MMR call redirection is unavailable on Azure Virtual Desktop for Azure US Government and on Windows 365 for GCC, GCC-High or DoD, because Microsoft does not support MMR there[^16].

See also [Troubleshooting Amazon Connect softphone and CCP audio](https://warmtransfer.net/knowledge/amazon-connect-ccp-audio-troubleshooting).

## Applicability

Applies to: Cisco Webex App, Microsoft Teams, Zoom Workplace VDI, AWS Amazon Connect, and Microsoft Azure Virtual Desktop. Deployments: vdi. Sources checked 2026-10-02. Microsoft lists 2 causes of Teams VDI error 16389 as fixed in plugin version 2026.29.1.4, so plugins earlier than that version are affected[^44]. Teams VDI event logging in Event Viewer applies to Teams 24123.X.X.X or higher[^39]. Webex App VDI ends support for Windows versions earlier than Windows 10 1809 and macOS versions earlier than macOS 13 Ventura starting with release 46.10[^54]. The Chrome Local Network Access issue affects Chrome 147 and Edge 147 and later[^10]. Amazon Connect MMR call redirection excludes Azure US Government and Windows 365 GCC, GCC-High and DoD[^16].

## What remains uncertain

Minimum Citrix VDA, Citrix Workspace app, Omnissa and AVD versions for Teams VDI 2.0 are not covered by the sources below. Error 2004 indicates only that the Citrix Workspace app version on the endpoint is unsupported and must be upgraded[^34]. Error 4390 is documented as a SlimCore MSIX error on thin clients that use Unified Write Filters and RAM-disk overlays[^38]. Broader Unified Write Filter configuration guidance for Teams VDI 2.0 is not covered by the sources below. The end-of-support date for WebRTC-based Teams VDI optimization is not covered by the sources below. Webex says new Webex App VDI versions stay backward compatible with earlier thin-client plugins[^55]. The specific plugin compatibility window for regular Webex App VDI releases is not covered by the sources below. Softphone rules on Citrix for other contact-center platforms, including running a WebRTC softphone and a separate softphone in the same browser, are not covered by the sources below. A raw-text verification of the versions and status strings on Citrix's own Teams optimization page is not covered by the sources below. Omnissa Horizon guidance for Teams optimization, including Media Not Connected and a hosts-file cause, is not covered by the sources below. The behavior of other cloud contact-center WebRTC softphones in VDI is not covered by the sources below. Headset HID call control under VDI for each vendor is not covered by the sources below.

## Sources

[^1]: Microsoft's AVD troubleshooting page says Teams media optimization is supported only with the Remote Desktop client on Windows 10 or later or macOS 10.14 or later, and not for Teams running as a RemoteApp on macOS endpoints. Source: [Troubleshoot Microsoft Teams on Azure Virtual Desktop](https://learn.microsoft.com/en-us/azure/virtual-desktop/troubleshoot-teams), Section 'Known issues and limitations' > 'Client deployment, installation, and set up'. Checked 2026-10-02.
[^2]: The Remote Desktop WebRTC Redirector Service is not pre-installed on the Windows 11 Enterprise multi-session + Microsoft 365 Apps marketplace images 23H2 (build 22631.3593) and 22H2 (build 22621.3593); the fix is to redeploy from a newer image or install the service separately. Source: [Troubleshoot Microsoft Teams on Azure Virtual Desktop](https://learn.microsoft.com/en-us/azure/virtual-desktop/troubleshoot-teams), Section 'Remote Desktop WebRTC Redirector Service'. Checked 2026-10-02.
[^3]: Citrix says 'Citrix HDX Not Connected' in Teams means the Citrix API loaded but an error occurred in the VDA services or Citrix Workspace app; no legend at all means Teams failed to load the Citrix API. Source: [Optimization for Microsoft Teams](https://docs.citrix.com/en-us/citrix-virtual-apps-desktops/multimedia/opt-ms-teams.html), Verification section (heading as reported by summariser). Checked 2026-10-02.
[^4]: With Citrix's WebRTC-based Teams optimization, a successful redirection shows 'Citrix HDX Optimized' under About > Version in Teams. Source: [Optimization for Microsoft Teams](https://docs.citrix.com/en-us/citrix-virtual-apps-desktops/multimedia/opt-ms-teams.html), Verification section (heading as reported by summariser). Checked 2026-10-02.
[^5]: In Citrix's WebRTC-based Teams optimization, HdxRtcEngine.exe on the endpoint's Citrix Workspace app processes the media, and media flows between the endpoint and peers or conference servers rather than through the VDA. Source: [Optimization for Microsoft Teams](https://docs.citrix.com/en-us/citrix-virtual-apps-desktops/multimedia/opt-ms-teams.html), How it works / architecture section (heading as reported by summariser). Checked 2026-10-02.
[^6]: The Citrix 'Microsoft Teams redirection' policy is on by default. Source: [Optimization for Microsoft Teams](https://docs.citrix.com/en-us/citrix-virtual-apps-desktops/multimedia/opt-ms-teams.html), Enable optimization section (heading as reported by summariser). Checked 2026-10-02.
[^7]: For Citrix-optimized Teams, if UDP is blocked between the endpoint and Microsoft 365 TURN servers, the endpoint media engine falls back to TCP/TLS 443, and ICE failures can cause one-way audio or call setup failures. Source: [Optimization for Microsoft Teams](https://docs.citrix.com/en-us/citrix-virtual-apps-desktops/multimedia/opt-ms-teams.html), Network requirements / troubleshooting section (heading as reported by summariser). Checked 2026-10-02.
[^8]: When VDIPlatform is set to AZURE, the Amazon Connect CCP uses Azure (MMR) audio optimization only and does not fall back to browser audio; if VDIPlatform is unset, Connect auto-detects and enables Azure optimization when available. Source: [Optimize Connect Customer audio for Azure Virtual Desktop and Windows 365](https://docs.aws.amazon.com/connect/latest/adminguide/using-ccp-vdi-azure-step-by-step.html), Section 'Configure the VDI platform parameter', Note. Checked 2026-10-02.
[^9]: To verify Amazon Connect MMR call redirection on AVD/Windows 365, AWS says navigator.mediaDevices.isCallRedirectionEnabled should return true, and a call should still work with microphone permission disabled for the CCP domain in the virtual desktop browser. Source: [Optimize Connect Customer audio for Azure Virtual Desktop and Windows 365](https://docs.aws.amazon.com/connect/latest/adminguide/using-ccp-vdi-azure-step-by-step.html), Section 'Verify the media flow between the local device and Connect Customer', steps 2 and 3. Checked 2026-10-02.
[^10]: AWS warns that Local Network Access prompts introduced in Chrome 147 and Edge 147 can block Citrix UCSDK optimization, so Amazon Connect audio optimization does not work until the workaround in Citrix article CTX696569 is applied. Source: [Optimize Connect Customer audio for Citrix cloud desktops](https://docs.aws.amazon.com/connect/latest/adminguide/using-ccp-vdi-citrix-step-by-step.html), Section 'System requirements', Important notice. Checked 2026-10-02.
[^11]: To fix dead air and dropped calls during mute in older Citrix Workspace app versions, AWS says to set VDIPlatform to CITRIX_413 instead of CITRIX and use CWA 2508.10 or later on Windows, Linux and Mac (2507 or later on ChromeOS); CWA 2305 is the absolute minimum. Source: [Optimize Connect Customer audio for Citrix cloud desktops](https://docs.aws.amazon.com/connect/latest/adminguide/using-ccp-vdi-citrix-step-by-step.html), Section 'System requirements', 'Upgrade Citrix Workspace App to improve call quality' and 'Citrix Workspace Application Version'. Checked 2026-10-02.
[^12]: For Citrix VDI, Amazon Connect audio optimization uses a custom CCP built with the Connect RTC JavaScript library, which integrates with the Citrix Unified Communications SDK (UCSDK) to redirect media to the local desktop. Source: [Scenario and deployment approaches in Connect Customer](https://docs.aws.amazon.com/connect/latest/adminguide/scenario-deployment-approaches.html), Section 'Citrix VDI with Connect Customer audio optimization'. Checked 2026-10-02.
[^13]: To confirm that Amazon Connect audio is redirected under Citrix, AWS says to check Task Manager on the agent's thin client for the running HDX service. Source: [Optimize Connect Customer audio for Citrix cloud desktops](https://docs.aws.amazon.com/connect/latest/adminguide/using-ccp-vdi-citrix-step-by-step.html), Section 'Confirm media flows between thin client and Connect Customer during the call'. Checked 2026-10-02.
[^14]: The Citrix UCSDK is not enabled for Amazon Connect by default: the admin must add a ProcessWhitelist REG_MULTI_SZ value (Chrome.exe, msedge.exe) under HKLM\SOFTWARE\WOW6432Node\Citrix\WebSocketService on the VDA and restart CitrixHdxWebSocketService. Source: [Optimize Connect Customer audio for Citrix cloud desktops](https://docs.aws.amazon.com/connect/latest/adminguide/using-ccp-vdi-citrix-step-by-step.html), Section 'System requirements' > 'Citrix Server Setup'. Checked 2026-10-02.
[^15]: The MMR extension redirects calls only for domains on its AllowedCallRedirectionSites allowlist; Connect-hosted CCPs are on the default list, but a custom CCP on the customer's own domain must be added by policy. Source: [Optimize Connect Customer audio for Azure Virtual Desktop and Windows 365](https://docs.aws.amazon.com/connect/latest/adminguide/using-ccp-vdi-azure-step-by-step.html), Section 'System requirements' > 'Call redirection allowlist'. Checked 2026-10-02.
[^16]: Amazon Connect MMR call redirection is unavailable on Azure Virtual Desktop for Azure US Government and on Windows 365 for GCC, GCC-High or DoD, because Microsoft does not support MMR there. Source: [Optimize Connect Customer audio for Azure Virtual Desktop and Windows 365](https://docs.aws.amazon.com/connect/latest/adminguide/using-ccp-vdi-azure-step-by-step.html), Section 'Limitations'. Checked 2026-10-02.
[^17]: Amazon Connect VDI audio optimization does not support the native CCP; with the native CCP, media keeps flowing through the browser inside the VM. Source: [Optimize Connect Customer audio for Omnissa cloud desktops](https://docs.aws.amazon.com/connect/latest/adminguide/using-ccp-vdi-omnissa-step-by-step.html), Section 'Limitations'. Checked 2026-10-02.
[^18]: Amazon Connect does not support Voice Focus noise suppression in VDI environments because it is incompatible with audio redirection. Source: [Optimize Connect Customer audio for Azure Virtual Desktop and Windows 365](https://docs.aws.amazon.com/connect/latest/adminguide/using-ccp-vdi-azure-step-by-step.html), Section 'Limitations'. Checked 2026-10-02.
[^19]: Amazon Connect audio optimization with Omnissa needs Horizon Client and agent 8.15.0 (Horizon 2503) or later and, for now, supports only Windows agent workstations; 2503 is the first version to support ICE restart. Source: [Optimize Connect Customer audio for Omnissa cloud desktops](https://docs.aws.amazon.com/connect/latest/adminguide/using-ccp-vdi-omnissa-step-by-step.html), Section 'System requirements' > 'Omnissa Horizon Client Version'. Checked 2026-10-02.
[^20]: When an Amazon Connect CCP sets VDIPlatform to "OMNISSA", it does not fall back to standard browser audio if Omnissa optimization fails, so calls fail when an agent opens that CCP outside the Omnissa VM. Source: [Optimize Connect Customer audio for Omnissa cloud desktops](https://docs.aws.amazon.com/connect/latest/adminguide/using-ccp-vdi-omnissa-step-by-step.html), Section 'Required code changes on your custom CCP', step 2 Important note. Checked 2026-10-02.
[^21]: To verify Amazon Connect Omnissa optimization, AWS says to check that the Horizon WebRTC browser extension icon is blue (Ready), that html5server.exe and wsnm.exe stay running during calls, and that the browser console logs 'RTC.js is using OmnissaVDIStrategy'. Source: [Optimize Connect Customer audio for Omnissa cloud desktops](https://docs.aws.amazon.com/connect/latest/adminguide/using-ccp-vdi-omnissa-step-by-step.html), Section 'How to verify the media flow between thin client and Amazon Connect during the call', steps 1, 3 and 5. Checked 2026-10-02.
[^22]: With Omnissa, the Amazon Connect Salesforce CTI Adapter does not support VDI platform detection, so its media goes through the VM's browser rather than being processed on the client. Source: [Optimize Connect Customer audio for Omnissa cloud desktops](https://docs.aws.amazon.com/connect/latest/adminguide/using-ccp-vdi-omnissa-step-by-step.html), Section 'Limitations'. Checked 2026-10-02.
[^23]: When the VDI client has no local browser, Amazon Connect describes a single CCP running media on the VDI server, usually with UDP audio enabled in the VDI OS, and says the model needs extensive testing to tune VDI server parameters. Source: [Scenario and deployment approaches in Connect Customer](https://docs.aws.amazon.com/connect/latest/adminguide/scenario-deployment-approaches.html), Section 'Virtual desktop infrastructure (VDI)' > 'VDI client without local browser access'. Checked 2026-10-02.
[^24]: One common Amazon Connect VDI model uses a custom CCP with no media on the virtual desktop for signalling and call control, while media is handled on the agent's local desktop by a standard CCP. Source: [Scenario and deployment approaches in Connect Customer](https://docs.aws.amazon.com/connect/latest/adminguide/scenario-deployment-approaches.html), Section 'Virtual desktop infrastructure (VDI)' > 'VDI client with local browser access'. Checked 2026-10-02.
[^25]: Optimized Amazon Connect VDI audio needs the VDI server to reach Amazon Connect on TCP 443 and the agent's thin client or local device to reach Amazon Connect softphone media on UDP 3478. Source: [Optimize Connect Customer audio for Citrix cloud desktops](https://docs.aws.amazon.com/connect/latest/adminguide/using-ccp-vdi-citrix-step-by-step.html), Section 'System requirements' > 'Networking/Firewall Configurations'. Checked 2026-10-02.
[^26]: On an Azure Virtual Desktop endpoint, Microsoft says to confirm that the msrdc.exe process has loaded MsTeamsPluginAvd.dll. Source: [Troubleshooting the VDI 2.0 solution for Teams](https://learn.microsoft.com/en-us/microsoftteams/vdi-2-troubleshooting), Introductory troubleshooting step 2.3. Checked 2026-10-02.
[^27]: On a Citrix endpoint, Microsoft says to use Process Explorer to confirm that the wfica32.exe process has loaded MsTeamsPluginCitrix.dll. Source: [Troubleshooting the VDI 2.0 solution for Teams](https://learn.microsoft.com/en-us/microsoftteams/vdi-2-troubleshooting), Introductory troubleshooting step 2.4. Checked 2026-10-02.
[^28]: For Teams SlimCore optimization on Citrix, the Virtual Channel Allow List policy in Citrix Studio must allow the custom virtual channels MSTEAMS, MSTEAM1 and MSTEAM2; if they are blocked, Teams logs error 2003 (deployErrc 16026). Source: [Troubleshooting the VDI 2.0 solution for Teams](https://learn.microsoft.com/en-us/microsoftteams/vdi-2-troubleshooting), Introductory troubleshooting step 1; Connection error table row 2003 / 16026. Checked 2026-10-02.
[^29]: In Vdi_debug.txt, connectedStack "local" means the virtual channel connection failed and the user is in fallback mode; "remote" means Teams reached the endpoint over the virtual channel, but it does not guarantee the calling stack initialised. Source: [Troubleshooting the VDI 2.0 solution for Teams](https://learn.microsoft.com/en-us/microsoftteams/vdi-2-troubleshooting), Section 'Vdi_debug.txt', bullets under vdiConnectedState. Checked 2026-10-02.
[^30]: A Teams VDI deploy error (deployErrc) happens while the plugin downloads the SlimCore MSIX from Microsoft's CDN and stages it through the App Readiness Service; a load error (loadErrc) happens while starting MsTeamsVdi.exe and opening an RPC to it. Source: [Troubleshooting the VDI 2.0 solution for Teams](https://learn.microsoft.com/en-us/microsoftteams/vdi-2-troubleshooting), Section 'Connection error', opening paragraph. Checked 2026-10-02.
[^31]: Teams VDI error 1260 (deployErrc 10083, Event Viewer 0x800704EC) usually means Windows Package Manager cannot install the SlimCore MSIX, often because of AppLocker; the fix is to disable AppLocker or add an exception for SlimCoreVdi packages. Source: [Troubleshooting the VDI 2.0 solution for Teams](https://learn.microsoft.com/en-us/microsoftteams/vdi-2-troubleshooting), Section 'Connection error', table row 1260 / 10083. Checked 2026-10-02.
[^32]: Microsoft names the BlockNonAdminUserInstall policy (including the Intune 'Block non admin user install' setting) as the most likely cause of Teams VDI error 16389 (E_FAIL); when it applies, SlimCore registration aborts and the call falls back to non-optimized or, where available, WebRTC. Source: [Troubleshooting the VDI 2.0 solution for Teams](https://learn.microsoft.com/en-us/microsoftteams/vdi-2-troubleshooting), Section 'Error 16389', note and cause table (BlockNonAdminUserInstall: Very high). Checked 2026-10-02.
[^33]: Teams VDI error 1722 (RPC server unavailable) commonly occurs on slow thin clients overloaded at logon or with delayed AppX registration; Microsoft says to treat it as transient if optimization eventually succeeds. Source: [Troubleshooting the VDI 2.0 solution for Teams](https://learn.microsoft.com/en-us/microsoftteams/vdi-2-troubleshooting), Section 'Connection error', table row 1722 / 13779. Checked 2026-10-02.
[^34]: Teams VDI error 2004 (deployErrc 16032) means the Citrix Workspace app version on the endpoint is not supported and must be upgraded. Source: [Troubleshooting the VDI 2.0 solution for Teams](https://learn.microsoft.com/en-us/microsoftteams/vdi-2-troubleshooting), Section 'Connection error', table row 2004 / 16032. Checked 2026-10-02.
[^35]: When Teams runs as a Citrix published app or an AVD/Windows 365 RemoteApp (error 2005 / 16043), Teams does not load SlimCore and the user is always optimized with WebRTC. Source: [Troubleshooting the VDI 2.0 solution for Teams](https://learn.microsoft.com/en-us/microsoftteams/vdi-2-troubleshooting), Section 'Connection error', table row 2005 / 16043. Checked 2026-10-02.
[^36]: Teams VDI codes 3000 (deployErrc 24002, SlimCore deployment not needed) and 3001 (24010, SlimCore already loaded) are not errors; they show the user is on the SlimCore architecture. Source: [Troubleshooting the VDI 2.0 solution for Teams](https://learn.microsoft.com/en-us/microsoftteams/vdi-2-troubleshooting), Section 'Connection error', table rows 3000 / 24002 and 3001 / 24010. Checked 2026-10-02.
[^37]: Teams VDI error 403 (deployErrc 3227) means something on the network, typically a proxy, is blocking the SlimCore MSIX download; 404 (3235) points to a missing CDN package or proxy interference. Source: [Troubleshooting the VDI 2.0 solution for Teams](https://learn.microsoft.com/en-us/microsoftteams/vdi-2-troubleshooting), Section 'Connection error', table rows 403 / 3227 and 404 / 3235. Checked 2026-10-02.
[^38]: Teams VDI error 4390 (ERROR_NOT_A_REPARSE_POINT) is a SlimCore MSIX error seen on thin clients that use Unified Write Filters and RAM-disk overlays. Source: [Troubleshooting the VDI 2.0 solution for Teams](https://learn.microsoft.com/en-us/microsoftteams/vdi-2-troubleshooting), Section 'Connection error', table row 4390. Checked 2026-10-02.
[^39]: Teams VDI connect and disconnect events are logged in the VM's Windows Logs\Application under source 'Microsoft Teams VDI', Event ID 0; filtering by that source first requires registering it with New-EventLog from an elevated PowerShell. Source: [Troubleshooting the VDI 2.0 solution for Teams](https://learn.microsoft.com/en-us/microsoftteams/vdi-2-troubleshooting), Section 'Using Event Viewer on the VM for troubleshooting'. Checked 2026-10-02.
[^40]: When Teams SlimCore-based VDI optimization fails to initialise, users can end up in a fallback mode reported as "SlimCore Media Not Connected" (server-side rendering), distinct from being optimized on the legacy WebRTC stack. Source: [Troubleshooting the VDI 2.0 solution for Teams](https://learn.microsoft.com/en-us/microsoftteams/vdi-2-troubleshooting), Introductory paragraph above 'Teams Client Health dashboard'. Checked 2026-10-02.
[^41]: If a Teams VDI user sees "Azure Virtual Desktop Media Optimized" or "Citrix HDX Optimized" instead of SlimCore, they are on the legacy WebRTC stack, and error codes 2000 (No Plugin) and 2003 (Virtual Channel not allowed) are the most likely causes. Source: [Troubleshooting the VDI 2.0 solution for Teams](https://learn.microsoft.com/en-us/microsoftteams/vdi-2-troubleshooting), Introductory bullet 'Not optimized with SlimCore and instead you see: Azure Virtual Desktop Media Optimized / Citrix HDX Optimized'. Checked 2026-10-02.
[^42]: Teams VDI logs are collected by pressing Ctrl+Alt+Shift+1 in Teams on the VM; Vdi_debug.txt inside the resulting PROD-WebLogs zip is the main file for VDI information. Source: [Troubleshooting the VDI 2.0 solution for Teams](https://learn.microsoft.com/en-us/microsoftteams/vdi-2-troubleshooting), Sections 'Teams logs for VDI' and 'Vdi_debug.txt (main file for VDI-related information)'. Checked 2026-10-02.
[^43]: When Teams shows "Azure Virtual Desktop SlimCore Media Not Connected" or "Citrix SlimCore Media Not Connected", Microsoft names MSIX or AppX deployment errors as the most likely cause. Source: [Troubleshooting the VDI 2.0 solution for Teams](https://learn.microsoft.com/en-us/microsoftteams/vdi-2-troubleshooting), Introductory bullet 'Not optimized with SlimCore and instead you see: ... SlimCore Media Not Connected'. Checked 2026-10-02.
[^44]: Microsoft lists SlimCore Host/Framework registration mismatch and the shared-kiosk multi-user registration issue as causes of Teams VDI error 16389 that are fixed in plugin version 2026.29.1.4. Source: [Troubleshooting the VDI 2.0 solution for Teams](https://learn.microsoft.com/en-us/microsoftteams/vdi-2-troubleshooting), Section 'Error 16389', cause table rows 'SlimCore Host/FWK registration mismatch' and 'Shared kiosk multi-user registration issue'. Checked 2026-10-02.
[^45]: Vdi_debug.txt records the version-mismatch evidence for Teams VDI: bridgeVersion (tied to the Teams client on the VM), remoteSlimcoreVersion (SlimCore on the endpoint), pluginVersion (plugin in the remote desktop client) and rdClientVersion. Source: [Troubleshooting the VDI 2.0 solution for Teams](https://learn.microsoft.com/en-us/microsoftteams/vdi-2-troubleshooting), Section 'Vdi_debug.txt', bullets under vdiVersionInfo. Checked 2026-10-02.
[^46]: The first time Teams detects the endpoint plugin, it needs one restart to move from WebRTC to SlimCore optimization. Source: [Troubleshooting the VDI 2.0 solution for Teams](https://learn.microsoft.com/en-us/microsoftteams/vdi-2-troubleshooting), Introductory troubleshooting step 3. Checked 2026-10-02.
[^47]: The Teams client health page in the Teams admin center lists 'VDI optimization failure' errors with descriptions and remediation actions on its Issues tab. Source: [Troubleshooting the VDI 2.0 solution for Teams](https://learn.microsoft.com/en-us/microsoftteams/vdi-2-troubleshooting), Section 'Teams Client Health dashboard'. Checked 2026-10-02.
[^48]: In 2026, Webex App VDI slow channel releases are 46.3 (March), 46.6 (June) and 46.10 (October), with no April (46.4) release. Source: [Webex App | VDI release notes](https://help.webex.com/en-us/article/ntp1us7/Webex-App-%7C-VDI-release-notes), Release notes, slow channel statement. Checked 2026-10-02.
[^49]: For Webex App VDI on Citrix with a virtual channel allow list policy, the Webex virtual channel CSCOTM must be added with the process path C:\Program Files\Cisco Spark\dependencies\teamshvdagent.exe. Source: [Deployment guide for Webex App for Virtual Desktop Infrastructure (VDI)](https://help.webex.com/en-us/article/19qt9v/Deployment-guide-for-Webex-App-for-Virtual-Desktop-Infrastructure-(VDI)), Section 'Hosted virtual desktop'. Checked 2026-10-02.
[^50]: Webex App VDI fallback mode gives short-term support for basic audio and video calls when the virtual channel cannot be established, at lower call quality and without the full feature set. Source: [Deployment guide for Webex App for Virtual Desktop Infrastructure (VDI)](https://help.webex.com/en-us/article/19qt9v/Deployment-guide-for-Webex-App-for-Virtual-Desktop-Infrastructure-(VDI)), Section 'Fallback mode'. Checked 2026-10-02.
[^51]: Cisco says that without VDI optimization, Webex App calling, meetings and video on a hosted virtual desktop are not supported, because media would hairpin through the data center. Source: [Deployment guide for Webex App for Virtual Desktop Infrastructure (VDI)](https://help.webex.com/en-us/article/19qt9v/Deployment-guide-for-Webex-App-for-Virtual-Desktop-Infrastructure-(VDI)), Section 'Webex App for VDI overview'. Checked 2026-10-02.
[^52]: In Webex App VDI, a correctly installed and working thin-client plugin makes the app's Health Check show a connected status for the Virtual Channel. Source: [Deployment guide for Webex App for Virtual Desktop Infrastructure (VDI)](https://help.webex.com/en-us/article/19qt9v/Deployment-guide-for-Webex-App-for-Virtual-Desktop-Infrastructure-(VDI)), Section 'Install the Webex App VDI plugin on Windows thin client systems'. Checked 2026-10-02.
[^53]: On Linux thin clients, the Webex Meetings VDI plugin must be installed alongside the Webex App VDI plugin; Windows and macOS use a bundled installer. Source: [Deployment guide for Webex App for Virtual Desktop Infrastructure (VDI)](https://help.webex.com/en-us/article/19qt9v/Deployment-guide-for-Webex-App-for-Virtual-Desktop-Infrastructure-(VDI)), Thin client plugin installation section for Linux. Checked 2026-10-02.
[^54]: Starting with the October 46.10 release, Webex App VDI ends support for Windows versions earlier than Windows 10 1809 and macOS versions earlier than macOS 13 Ventura. Source: [Webex App | VDI release notes](https://help.webex.com/en-us/article/ntp1us7/Webex-App-%7C-VDI-release-notes), Release notes, end-of-support announcements. Checked 2026-10-02.
[^55]: New Webex App VDI versions are released every two months and stay backward compatible with earlier thin-client VDI plugins. Source: [Deployment guide for Webex App for Virtual Desktop Infrastructure (VDI)](https://help.webex.com/en-us/article/19qt9v/Deployment-guide-for-Webex-App-for-Virtual-Desktop-Infrastructure-(VDI)), Section 'Architecture'. Checked 2026-10-02.
[^56]: For Webex App VDI slow channel versions, the latest release supports the latest plugin, four previous plugin releases, and the two previous slow channel plugin releases. Source: [Webex App | VDI release notes](https://help.webex.com/en-us/article/ntp1us7/Webex-App-%7C-VDI-release-notes), Release notes, slow channel / compatibility statement. Checked 2026-10-02.
[^57]: Cisco documents a 'VDIFallbackNoMRA' registry key on the hosted virtual desktop that lets users stay in VDI optimized mode (a fallback-mode control). Source: [Deployment guide for Webex App for Virtual Desktop Infrastructure (VDI)](https://help.webex.com/en-us/article/19qt9v/Deployment-guide-for-Webex-App-for-Virtual-Desktop-Infrastructure-(VDI)), Section 'Fallback mode'. Checked 2026-10-02.
[^58]: For Teams, Webex App, Zoom and Amazon Connect, the vendor fallback or unoptimized mode puts media processing back on the virtual desktop (the hairpin), so 'optimized but degraded' and 'fallback' need different evidence: the vendor's own status indicator, not call quality alone (inferred). Source: [Deployment guide for Webex App for Virtual Desktop Infrastructure (VDI)](https://help.webex.com/en-us/article/19qt9v/Deployment-guide-for-Webex-App-for-Virtual-Desktop-Infrastructure-(VDI)), Section 'Webex App for VDI overview' (hairpin), combined with the fallback descriptions in the corroborating sources. Checked 2026-10-02.
[^59]: In Zoom VDI fallback mode, all media is sent from the remote device to the virtual desktop instead of directly to meeting participants. Source: [VDI installation and connectivity](https://support.zoom.com/hc/en/article?id=zm_kb&sysparm_article=KB0060901), Fallback mode description in 'VDI installation and connectivity'. Checked 2026-10-02.
[^60]: The Zoom VDI plugin on the thin client must always be the same version as, or lower than, the Zoom VDI client on the virtual desktop. Source: [VDI installation and connectivity](https://support.zoom.com/hc/en/article?id=zm_kb&sysparm_article=KB0060901), Plugin version guidance in 'VDI installation and connectivity'. Checked 2026-10-02.