# UCaaS platforms compared

Canonical: https://warmtransfer.net/knowledge/ucaas-platform-comparison

Last verified: 2026-09-30

Enterprise unified communications as a service (UCaaS) platforms provide cloud-delivered voice, survivability, hardware integration, and contact center connectivity options[^19][^45][^46][^7]. Across [Microsoft Teams Phone](https://warmtransfer.net/knowledge/microsoft-teams-phone), Webex Calling, Zoom Phone, RingCentral, and 8x8, architectures vary significantly across PSTN routing methods, survivable branch mechanics, endpoint firmware, and contact center integration models[^19][^45][^30][^54][^2].

## PSTN connectivity models

Microsoft documents 4 PSTN connectivity models for Teams Phone: Microsoft Teams Calling Plan, Operator Connect, Teams Phone Mobile, and [Direct Routing](https://warmtransfer.net/knowledge/microsoft-teams-direct-routing)[^19]. A single Teams tenant can use more than 1 PSTN connectivity model at the same time, such as different models for different user personas or regions[^20]. With Microsoft Teams Calling Plan, Microsoft is the operator supplying PSTN access, numbers, and support, with a stated 99.999% reliability SLA[^14]. Operator Connect uses certified third-party operators already integrated with Microsoft; the customer enables the operator in the Teams admin center and assigns that operator's numbers to users[^22]. Teams Phone Mobile supports user phone numbers only and does not support Audio Conferencing numbers or voice application numbers for auto attendants and call queues[^21]. Teams Direct Routing works with any PSTN operator but requires a certified [session border controller](https://warmtransfer.net/knowledge/cisco-unified-border-element) procured and managed by the customer, an integrator, or a Direct-Routing-as-a-Service provider[^18].

Webex Calling offers 3 PSTN options when adding a location: Cisco PSTN (Cisco Calling Plan), Cloud Connected PSTN from a certified provider, and premises-based PSTN through a Local Gateway[^45]. In Webex Calling the PSTN option is chosen per location and each location has exactly 1 PSTN option, although different locations in the same organization can use different options[^40].

Zoom Phone BYOC Cloud Peering (BYOC-C) connects 1 or more carriers to Zoom Phone data centers without requiring on-premises hardware[^46]. Zoom states that its Provider Exchange cloud-peering network collectively covers over 90 countries with PSTN connectivity[^49]. Zoom documents combining a cloud peering provider with the Zoom Phone Native service model to extend international coverage[^48].

RingCentral BYOC lets customers receive RingEX cloud PBX functionality while connecting their existing local voice carrier[^7]. RingCentral's BYOC+ partner page lists Bouygues, China Mobile, Avoxi, Colt, and Enreach as certified carrier partners[^9]. RingCentral's BYOC service description disclaims emergency calling in countries where RingCentral has not enabled local service and requires the customer to provide alternative emergency calling arrangements there[^8].

8x8 Voice for Microsoft Teams is a cloud-to-cloud Direct Routing integration in which 8x8 provides PSTN service to Teams users[^2]. It requires each user to hold a Microsoft Phone System (Teams Phone) license and an 8x8 X Series license (X1-X8), and users must not be assigned Microsoft Calling Plans[^3]. It does not support US Government GCC High or DoD tenants[^4]. Alternatively, the 8x8 for Microsoft Teams app lets users with a basic Teams license place calls and send SMS, MMS, and faxes on their 8x8 number without a Teams Phone license, provided Presence Sync is enabled and 8x8 Work is installed for PSTN calls[^1].

## Branch site survivability

WarmTransfer's reading of the sources is that across Teams Phone, Webex Calling, and Zoom Phone, documented site survivability depends on an on-premises component plus a locally terminated PSTN trunk; cloud-only carrier options such as Teams Calling Plan, Cisco PSTN, or Zoom Native numbers do not by themselves give PSTN service during a site outage[^13].

The Teams Survivable Branch Appliance (SBA) is Microsoft code distributed to SBC vendors, who embed it in SBC firmware or ship it to run on a separate VM or hardware, for Direct Routing branch sites[^30]. The Teams SBA requires the SBC to be configured for media bypass so the branch Teams client can send media directly to the SBC[^25]. SBA mode is supported on the Teams Windows desktop client, the Teams macOS desktop client, and Teams Phones; it activates only on physical machines, and virtual machines and web clients are not supported[^29]. In Teams SBA mode, users can make and receive PSTN calls through the local SBC, hold and resume, blind transfer, forward to a single number or Teams user, and redirect call queue or auto attendant calls to a local agent or alternative number[^27]. Emergency calls can be placed, but location information is not shared, and Emergency Call Routing Policies are not supported, meaning emergency calls fail unless the voice routing policy matches the emergency dial strings[^24]. Teams SBA configuration (appliances, branch survivability policy, user assignment) is done with Teams PowerShell cmdlets, as the Teams admin center does not yet support the Direct Routing SBA feature[^28]. If the tenant uses Continuous Access Evaluation (CAE) tokens, the Teams SBA is operational for only about 30 minutes; Microsoft gives disabling CAE for the tenant as an alternative[^23].

Webex Calling Site Survivability runs on a Cisco IOS XE router configured as a Survivability Gateway at the site[^42]. Endpoints can make internal site calls and external calls over local PSTN circuits or SIP trunks, with hold/resume, transfer, emergency calling, call forwarding, hunt groups, and a basic dial-by-extension auto attendant[^41]. Webex Calling Site Survivability does not support conferencing or 3-way calling, and video calls and Park, Barge, and Pickup softkeys are unavailable in survivability mode[^44]. The Survivability Gateway supports IPv4 only; IPv6 is not supported[^43].

Zoom Phone Local Survivability (ZPLS) is distributed as a Linux-based appliance run on an on-premises VMware ESXi host as a Zoom Node module[^54]. ZPLS configuration requires a Business, Education, or Enterprise account, a Zoom Node subscription, a Zoom Phone subscription, admin privileges, and Multiple Sites enabled for Zoom Phone[^56]. During ZPLS failover, Zoom Phone supports internal extension dialing, PSTN calling through an SBC, transfer, hold/resume, mute, DTMF, call park, and ad-hoc conferencing[^55]. However, only the BYOC carrier-provided PSTN number is functional; Zoom Native numbers can only be forwarded to BYOC numbers assigned to users in the same site[^52].

WarmTransfer's reading of the sources is that active-call behaviour at failover differs as documented: Microsoft says ongoing calls continue when Teams switches to the SBA, while Zoom says active calls are dropped when clients fail over to ZPLS, requiring users who hear a busy tone to re-establish the call[^26][^53][^5]. WarmTransfer's reading of the sources is that documented outage-mode feature sets also differ: Zoom lists ad-hoc conferencing under ZPLS, Webex excludes conferencing under Site Survivability, and the Teams SBA list does not include conferencing[^12].

## Endpoint and device ecosystems

[Teams SIP Gateway](https://warmtransfer.net/knowledge/teams-sip-gateway) lets compatible SIP devices act as Teams endpoints, including Cisco IP phones on multiplatform firmware and devices from Poly, Yealink, AudioCodes, Spectralink, Ascom, Gigaset, Algo, Alcatel-Lucent Enterprise, Snom, 2N, Avaya, and Axis[^34]. Cisco phones running enterprise firmware must be converted to multiplatform (MPP) firmware before they can be used with Teams SIP Gateway[^31]. Teams SIP Gateway requires the user to have a phone number with PSTN calling enabled, and it is not yet available for DoD tenants[^33]. SIP devices on Teams SIP Gateway can be call queue agents with restrictions, but SIP Gateway does not publish presence for devices, so presence-based routing is not supported for them[^32].

Cisco-supported devices for Webex Calling include the Cisco IP Phone 6800, 7800, and 8800 multiplatform series, 9800 series desk phones, DECT, Wi-Fi, and conference phones, ATAs, and Board, Room, and Desk collaboration devices[^38]. Webex Calling also supports listed third-party devices from vendors including Yealink, Poly, AudioCodes, and Snom, but using third-party devices requires approval from the customer's Cisco account manager before provisioning in Control Hub[^35].

The Zoom Phone supported-devices API reference lists devices from Cisco, Polycom, AudioCodes, and Yealink[^47].

## Contact center integration models

Microsoft defines 3 integration models for contact center providers with Teams: Connect (certified SBCs and Direct Routing), Extend (Teams client platform, Graph, and Cloud Communications APIs), and Unify (Azure Communication Services)[^17]. In the Teams Connect model, the contact center agent does not need a Teams Phone system license, but a Teams user receiving an agent transfer needs a phone system license and a phone number[^15]. In the Teams Extend model, Teams Phone carries all contact center calls and call control, and the contact center provider acts as a telephony carrier alongside Microsoft 365[^16].

Webex Customer Assist adds basic contact center capabilities to Webex Calling, including queue management, screen pop, analytics, and supervisor functions, delivered through the Webex App[^39]. A Webex Calling Standard license and a Customer Assist license cannot both be assigned to the same user[^36]. Cisco recommends [Webex Contact Center](https://warmtransfer.net/knowledge/webex-contact-center) rather than Customer Assist for customers needing sophisticated engagement, omnichannel routing, or large-scale high call volume deployments[^37].

For Zoom, a Zoom Contact Center agent can transfer a voice engagement to a Zoom Phone user, Zoom Phone call queue, or voicemail, as well as to contact center users, queues, and flows[^51]. Transferring from Zoom Contact Center to Zoom Phone users who have no Contact Center license requires the admin to enable contacts search[^50].

Since the November-December 2025 RingCX release, RingCX agents can search and directly call or transfer to users in federated RingEX directories across subsidiary and region-joined accounts[^11].

## See also

See also [CCaaS platforms compared](https://warmtransfer.net/knowledge/ccaas-platform-comparison).
See also [CUCM to Teams Phone migration](https://warmtransfer.net/knowledge/cucm-to-teams-phone).
See also [CUCM to Webex Calling migration](https://warmtransfer.net/knowledge/cucm-to-webex-calling).

## Applicability

Applies to: Microsoft Teams Phone, Microsoft Teams Phone Mobile, Microsoft Teams Calling Plan, Cisco Webex Calling, Cisco Webex Customer Assist, Zoom Phone Local Survivability, Zoom Phone, Zoom Contact Center, RingCentral RingEX, RingCentral RingEX BYOC+, RingCentral RingCX, 8x8 Voice for Microsoft Teams, 8x8 for Microsoft Teams app, and UCaaS. Deployments: hybrid and multi-tenant. Sources checked 2026-09-30. Teams SIP Gateway is not available for DoD tenants[^33]. 8x8 Voice for Microsoft Teams applies to the 8x8 X Series (X1-X8) and excludes US Government GCC High and DoD tenants[^3][^4].

## What remains uncertain

UCaaS per-user list pricing across platforms is not covered by the sources below. RingCentral Persist current availability and hardware is not covered by the sources below. An admin tooling and provisioning API comparison (Control Hub vs Teams admin center and PowerShell vs Zoom web portal vs RingCentral vs 8x8 Admin Console vs GoTo Admin) is not covered by the sources below. Any 8x8 local survivability option is not covered by the sources below. Webex Cloud Connected PSTN provider and country coverage is not covered by the sources below. Teams Operator Connect directory coverage by country is not covered by the sources below. Webex Dedicated Instance SRST survivability versus multi-tenant Site Survivability is not covered by the sources below. RingCentral BYOC certified SBC vendors are not covered by the sources below.

## Sources

[^1]: The 8x8 for Microsoft Teams app lets users with a basic Teams license place calls and send SMS, MMS and faxes on their 8x8 number without a Teams Phone license, provided Presence Sync is enabled and 8x8 Work is installed for PSTN calls. Source: [About the 8x8 for Microsoft Teams app for users](https://help.8x8.com/documentation/docs/about-the-8x8-for-microsoft-teams-app-for-users), sections 'Features' and 'Requirements'. Checked 2026-09-30.
[^2]: 8x8 Voice for Microsoft Teams is a cloud-to-cloud Direct Routing integration in which 8x8 provides PSTN service to Teams users. Source: [8x8 Voice for Microsoft Teams: Overview (administrator guide)](https://docs-alt.8x8.com/8x8WebHelp/8x8-voice-for-microsoft-teams-admin/Content/ms-teams/overview.htm), section 'Overview'. Checked 2026-09-30.
[^3]: 8x8 Voice for Microsoft Teams requires each user to hold a Microsoft Phone System (Teams Phone) license and an 8x8 X Series license (X1-X8), and users must not be assigned Microsoft Calling Plans. Source: [8x8 Voice for Microsoft Teams: Overview (administrator guide)](https://docs-alt.8x8.com/8x8WebHelp/8x8-voice-for-microsoft-teams-admin/Content/ms-teams/overview.htm), licensing requirements section. Checked 2026-09-30.
[^4]: 8x8 Voice for Microsoft Teams does not support US Government GCC High or DoD tenants. Source: [8x8 Voice for Microsoft Teams: Overview (administrator guide)](https://docs-alt.8x8.com/8x8WebHelp/8x8-voice-for-microsoft-teams-admin/Content/ms-teams/overview.htm), limitations / requirements section. Checked 2026-09-30.
[^5]: Active-call behaviour at failover differs as documented: Microsoft says ongoing calls continue when Teams switches to the SBA, while Zoom says active calls are dropped when clients fail over to ZPLS (inferred). Source: [Survivable Branch Appliance for Direct Routing](https://learn.microsoft.com/en-us/microsoftteams/direct-routing-survivable-branch-appliance), 'How it works' first paragraph; zoom-kb0058798-zpls-getting-started limitations section. Checked 2026-09-30.
[^6]: Under RingCentral BYOC, PSTN calls travel over the customer's local carrier through a gateway the customer purchases and owns. Source: [RingCentral Bring Your Own Carrier (BYOC) Service Description](https://www.ringcentral.com/legal/BYOC-service-description.html), service description body; exact lettered section not confirmed. Checked 2026-09-30.
[^7]: RingCentral BYOC lets customers receive RingEX cloud PBX functionality while connecting their existing local voice carrier. Source: [RingCentral Bring Your Own Carrier (BYOC) Service Description](https://www.ringcentral.com/legal/BYOC-service-description.html), Section A. Checked 2026-09-30.
[^8]: RingCentral's BYOC service description disclaims emergency calling in countries where RingCentral has not enabled local service and requires the customer to provide alternative emergency calling arrangements there. Source: [RingCentral Bring Your Own Carrier (BYOC) Service Description](https://www.ringcentral.com/legal/BYOC-service-description.html), Section D. Checked 2026-09-30.
[^9]: RingCentral's BYOC+ partner page lists Bouygues, China Mobile, Avoxi, Colt and Enreach as certified carrier partners. Source: [Become a Certified RingCentral BYOC+ Carrier Partner](https://www.ringcentral.com/partner/byoc.html), certified partners section. Checked 2026-09-30.
[^10]: RingCentral announced RingCentral Persist in March 2019 to keep PSTN inbound and outbound calling, emergency calling and extension-to-extension dialing working during an internet outage at a customer site. Source: [New RingCentral Persist Solution Maintains Services in the Event of Internet Outage at Customer Locations](https://www.ringcentral.com/whyringcentral/company/pressreleases/new-ringcentral-persist-solution-maintains-services-in-the-event-of-internet-outage-at-customer-locations.html), press release body, feature paragraphs. Checked 2026-09-30.
[^11]: Since the November-December 2025 RingCX release, RingCX agents can search and directly call or transfer to users in federated RingEX directories across subsidiary and region-joined accounts. Source: [RingCX Release Notes November - December 2025](https://support.ringcentral.com/release-notes/ringCX/release-notes-november-2025.html), section 'RCX Core', federated RingEX directories entry. Checked 2026-09-30.
[^12]: Documented outage-mode feature sets differ: Zoom lists ad-hoc conferencing under ZPLS, Webex excludes conferencing under Site Survivability, and the Teams SBA list does not include conferencing (inferred). Source: [Getting started with Zoom Phone Local Survivability service](https://support.zoom.com/hc/en/article?id=zm_kb&sysparm_article=KB0058798), combines ZPLS failover feature list; cisco-help-d68vi1-site-survivability 'Limitations and Restrictions'; ms-learn-direct-routing-sba 'How it works'. Checked 2026-09-30.
[^13]: Across Teams Phone, Webex Calling and Zoom Phone, documented site survivability depends on an on-premises component plus a locally terminated PSTN trunk; cloud-only carrier options such as Teams Calling Plan, Cisco PSTN or Zoom Native numbers do not by themselves give PSTN service during a site outage (inferred). Source: [Survivable Branch Appliance for Direct Routing](https://learn.microsoft.com/en-us/microsoftteams/direct-routing-survivable-branch-appliance), combines SBA 'Prerequisites'; cisco-help-d68vi1-site-survivability 'Supported Features & Components'; zoom-kb0063728-zpls-configure BYOC-number limitation. Checked 2026-09-30.
[^14]: With Microsoft Teams Calling Plan, Microsoft is the operator supplying PSTN access, numbers and support, with a stated 99.999% reliability SLA. Source: [PSTN connectivity options](https://learn.microsoft.com/en-us/microsoftteams/pstn-connectivity), section 'PSTN connectivity options', bullet 'Microsoft Teams Calling Plan'. Checked 2026-09-30.
[^15]: In the Teams Connect model the contact center agent does not need a Teams Phone system license, but a Teams user receiving an agent transfer needs a phone system license and a phone number. Source: [Teams Contact Center - Microsoft Teams](https://learn.microsoft.com/en-us/microsoftteams/teams-contact-center), tab 'Connect integration model', Note at end. Checked 2026-09-30.
[^16]: In the Teams Extend model, Teams Phone carries all contact center calls and call control, and the contact center provider acts as a telephony carrier alongside Microsoft 365. Source: [Teams Contact Center - Microsoft Teams](https://learn.microsoft.com/en-us/microsoftteams/teams-contact-center), tab 'Extend integration model', first paragraph. Checked 2026-09-30.
[^17]: Microsoft defines three integration models for contact center providers with Teams: Connect (certified SBCs and Direct Routing), Extend (Teams client platform, Graph and Cloud Communications APIs) and Unify (Azure Communication Services). Source: [Teams Contact Center - Microsoft Teams](https://learn.microsoft.com/en-us/microsoftteams/teams-contact-center), section 'Integration models for solution providers' and its three tabs. Checked 2026-09-30.
[^18]: Teams Direct Routing works with any PSTN operator but requires a certified session border controller procured and managed by the customer, an integrator or a Direct-Routing-as-a-Service provider. Source: [PSTN connectivity options](https://learn.microsoft.com/en-us/microsoftteams/pstn-connectivity), section 'PSTN connectivity options', bullet 'Direct Routing'. Checked 2026-09-30.
[^19]: Microsoft documents four PSTN connectivity models for Teams Phone: Microsoft Teams Calling Plan, Operator Connect, Teams Phone Mobile and Direct Routing. Source: [PSTN connectivity options](https://learn.microsoft.com/en-us/microsoftteams/pstn-connectivity), section 'PSTN connectivity options'. Checked 2026-09-30.
[^20]: A single Teams tenant can use more than one PSTN connectivity model at the same time, for example different models for different user personas or regions. Source: [PSTN connectivity options](https://learn.microsoft.com/en-us/microsoftteams/pstn-connectivity), section 'Use more than one PSTN connectivity type'. Checked 2026-09-30.
[^21]: Teams Phone Mobile supports user phone numbers only; it does not support Audio Conferencing numbers or voice application numbers for auto attendants and call queues. Source: [PSTN connectivity options](https://learn.microsoft.com/en-us/microsoftteams/pstn-connectivity), Note under 'PSTN connectivity options'; table 'PSTN connectivity types comparison' row 'Supports Audio Conferencing and voice application numbers'. Checked 2026-09-30.
[^22]: Operator Connect uses certified third-party operators already integrated with Microsoft; the customer enables the operator in the Teams admin center and assigns that operator's numbers to users. Source: [PSTN connectivity options](https://learn.microsoft.com/en-us/microsoftteams/pstn-connectivity), section 'PSTN connectivity options', bullet 'Operator Connect'. Checked 2026-09-30.
[^23]: If the tenant uses Continuous Access Evaluation tokens, the Teams SBA is operational for only about 30 minutes; Microsoft gives disabling CAE for the tenant as an alternative. Source: [Survivable Branch Appliance for Direct Routing](https://learn.microsoft.com/en-us/microsoftteams/direct-routing-survivable-branch-appliance), section 'Known issues and considerations', CAE bullet. Checked 2026-09-30.
[^24]: In Teams SBA mode emergency calls can be placed but location information is not shared, and Emergency Call Routing Policies are not supported, so emergency calls fail unless the voice routing policy matches the emergency dial strings. Source: [Survivable Branch Appliance for Direct Routing](https://learn.microsoft.com/en-us/microsoftteams/direct-routing-survivable-branch-appliance), section 'Known issues and considerations', list 'In SBA mode, the following user actions aren't supported'. Checked 2026-09-30.
[^25]: The Teams SBA requires the SBC to be configured for media bypass so the branch Teams client can send media directly to the SBC. Source: [Survivable Branch Appliance for Direct Routing](https://learn.microsoft.com/en-us/microsoftteams/direct-routing-survivable-branch-appliance), section 'Prerequisites', first bullet. Checked 2026-09-30.
[^26]: Microsoft states that during an internet outage the Teams client switches to the SBA automatically and ongoing calls continue without interruption. Source: [Survivable Branch Appliance for Direct Routing](https://learn.microsoft.com/en-us/microsoftteams/direct-routing-survivable-branch-appliance), section 'How it works', first paragraph. Checked 2026-09-30.
[^27]: In Teams SBA mode users can make and receive PSTN calls through the local SBC, hold and resume, blind transfer, forward to a single number or Teams user, and redirect call queue or auto attendant calls to a local agent or alternative number. Source: [Survivable Branch Appliance for Direct Routing](https://learn.microsoft.com/en-us/microsoftteams/direct-routing-survivable-branch-appliance), section 'How it works', list after 'When the Microsoft Teams client is in offline mode'. Checked 2026-09-30.
[^28]: Teams SBA configuration (appliances, branch survivability policy, user assignment) is done with Teams PowerShell cmdlets; the Teams admin center does not yet support the Direct Routing SBA feature. Source: [Survivable Branch Appliance for Direct Routing](https://learn.microsoft.com/en-us/microsoftteams/direct-routing-survivable-branch-appliance), section 'Configuration', paragraph after the numbered steps. Checked 2026-09-30.
[^29]: SBA mode is supported on the Teams Windows desktop client, the Teams macOS desktop client and Teams Phones; it activates only on physical machines, and virtual machines and web clients are not supported. Source: [Survivable Branch Appliance for Direct Routing](https://learn.microsoft.com/en-us/microsoftteams/direct-routing-survivable-branch-appliance), sections 'Supported Teams clients' and 'How it works'. Checked 2026-09-30.
[^30]: The Teams Survivable Branch Appliance is Microsoft code distributed to SBC vendors, who embed it in SBC firmware or ship it to run on a separate VM or hardware, for Direct Routing branch sites. Source: [Survivable Branch Appliance for Direct Routing](https://learn.microsoft.com/en-us/microsoftteams/direct-routing-survivable-branch-appliance), section 'Prerequisites', first paragraph. Checked 2026-09-30.
[^31]: Cisco phones running enterprise firmware must be converted to multiplatform (MPP) firmware before they can be used with Teams SIP Gateway. Source: [Plan SIP Gateway](https://learn.microsoft.com/en-us/microsoftteams/devices/sip-gateway-plan), table 'Compatible devices', Cisco row 'Remarks'. Checked 2026-09-30.
[^32]: SIP devices on Teams SIP Gateway can be call queue agents with restrictions; SIP Gateway does not publish presence for devices, so presence-based routing is not supported for them. Source: [Plan SIP Gateway](https://learn.microsoft.com/en-us/microsoftteams/devices/sip-gateway-plan), section 'Benefits of SIP Gateway', bullet 'Call Queues and voice apps support'. Checked 2026-09-30.
[^33]: Teams SIP Gateway requires the user to have a phone number with PSTN calling enabled, and SIP Gateway is not yet available for DoD tenants. Source: [Plan SIP Gateway](https://learn.microsoft.com/en-us/microsoftteams/devices/sip-gateway-plan), section 'Requirements to use SIP Gateway'. Checked 2026-09-30.
[^34]: Teams SIP Gateway lets compatible SIP devices act as Teams endpoints, including Cisco IP phones on multiplatform firmware and devices from Poly, Yealink, AudioCodes, Spectralink, Ascom, Gigaset, Algo, Alcatel-Lucent Enterprise, Snom, 2N, Avaya and Axis. Source: [Plan SIP Gateway](https://learn.microsoft.com/en-us/microsoftteams/devices/sip-gateway-plan), introduction and table 'Compatible devices'. Checked 2026-09-30.
[^35]: Webex Calling supports listed third-party devices from vendors including Yealink, Poly, AudioCodes and Snom, but using third-party devices requires approval from the customer's Cisco account manager before provisioning in Control Hub. Source: [Supported devices for Webex Calling](https://help.webex.com/en-us/article/qkwt4j/Supported-Devices-for-Webex-Calling), third-party devices section and its approval note. Checked 2026-09-30.
[^36]: A Webex Calling Standard license and a Customer Assist license cannot both be assigned to the same user. Source: [Webex Calling Customer Assist](https://help.webex.com/en-us/article/72sb3r/Webex-Calling-Customer-Assist), licensing section. Checked 2026-09-30.
[^37]: Cisco recommends Webex Contact Center rather than Customer Assist for customers needing sophisticated engagement, omnichannel routing or large-scale high call volume deployments. Source: [Webex Calling Customer Assist](https://help.webex.com/en-us/article/72sb3r/Webex-Calling-Customer-Assist), overview section, positioning paragraph. Checked 2026-09-30.
[^38]: Cisco-supported devices for Webex Calling include the Cisco IP Phone 6800, 7800 and 8800 multiplatform series, 9800 series desk phones, DECT, Wi-Fi and conference phones, ATAs and Board, Room and Desk collaboration devices. Source: [Supported devices for Webex Calling](https://help.webex.com/en-us/article/qkwt4j/Supported-Devices-for-Webex-Calling), section listing Cisco-supported devices. Checked 2026-09-30.
[^39]: Webex Customer Assist adds basic contact center capabilities to Webex Calling, including queue management, screen pop, analytics and supervisor functions, delivered through the Webex App. Source: [Webex Calling Customer Assist](https://help.webex.com/en-us/article/72sb3r/Webex-Calling-Customer-Assist), overview section. Checked 2026-09-30.
[^40]: In Webex Calling the PSTN option is chosen per location and each location has exactly one PSTN option, although different locations in the same organization can use different options. Source: [Configure Webex Calling for your organization](https://help.webex.com/en-us/article/njvdjf2/Configure-Webex-Calling-for-your-organization), section 'Add a location'. Checked 2026-09-30.
[^41]: During Webex Calling Site Survivability, endpoints can make internal site calls and external calls over local PSTN circuits or SIP trunks, with hold/resume, transfer, emergency calling, call forwarding, hunt groups and a basic dial-by-extension auto attendant. Source: [Site survivability for Webex Calling](https://help.webex.com/en-us/article/d68vi1/Site-Survivability-for-Webex-Calling), section 'Supported Features & Components'. Checked 2026-09-30.
[^42]: Webex Calling Site Survivability runs on a Cisco IOS XE router configured as a Survivability Gateway at the site. Source: [Site survivability for Webex Calling](https://help.webex.com/en-us/article/d68vi1/Site-Survivability-for-Webex-Calling), section 'Deployment Considerations' and platform table. Checked 2026-09-30.
[^43]: The Webex Calling Survivability Gateway supports IPv4 only; IPv6 is not supported. Source: [Site survivability for Webex Calling](https://help.webex.com/en-us/article/d68vi1/Site-Survivability-for-Webex-Calling), section 'Limitations and Restrictions'. Checked 2026-09-30.
[^44]: Webex Calling Site Survivability does not support conferencing or three-way calling, and video calls and Park, Barge and Pickup softkeys are unavailable in survivability mode. Source: [Site survivability for Webex Calling](https://help.webex.com/en-us/article/d68vi1/Site-Survivability-for-Webex-Calling), section 'Limitations and Restrictions'. Checked 2026-09-30.
[^45]: Webex Calling offers three PSTN options when adding a location: Cisco PSTN (Cisco Calling Plan), Cloud Connected PSTN from a certified provider, and premises-based PSTN through a Local Gateway. Source: [Configure Webex Calling for your organization](https://help.webex.com/en-us/article/njvdjf2/Configure-Webex-Calling-for-your-organization), section 'Add a location', step 5. Checked 2026-09-30.
[^46]: Zoom Phone BYOC Cloud Peering (BYOC-C) connects one or more carriers to Zoom Phone data centers without requiring on-premises hardware. Source: [Bring Your Own Carrier - Cloud Peering (BYOC-C) | Zoom Technical Library](https://library.zoom.com/zoom-workplace/zoom-phone/zoom-phone-bluepaper/overview/bring-your-own-carrier-cloud-peering-byoc-c), page body, service model description. Checked 2026-09-30.
[^47]: The Zoom Phone supported-devices API reference lists devices from Cisco, Polycom, AudioCodes and Yealink. Source: [Supported Devices List for Zoom Phone](https://developers.zoom.us/docs/api/references/phone-supported-devices/), manufacturer table. Checked 2026-09-30.
[^48]: Zoom documents combining a cloud peering provider with the Zoom Phone Native service model to extend international coverage. Source: [Bring Your Own Carrier - Cloud Peering (BYOC-C) | Zoom Technical Library](https://library.zoom.com/zoom-workplace/zoom-phone/zoom-phone-bluepaper/overview/bring-your-own-carrier-cloud-peering-byoc-c), page body, closing paragraph. Checked 2026-09-30.
[^49]: Zoom states that its Provider Exchange cloud-peering network collectively covers over 90 countries with PSTN connectivity. Source: [Bring Your Own Carrier - Cloud Peering (BYOC-C) | Zoom Technical Library](https://library.zoom.com/zoom-workplace/zoom-phone/zoom-phone-bluepaper/overview/bring-your-own-carrier-cloud-peering-byoc-c), page body, coverage sentence. Checked 2026-09-30.
[^50]: Transferring from Zoom Contact Center to Zoom Phone users who have no Contact Center license requires the admin to enable contacts search. Source: [Transferring a Zoom Contact Center voice engagement](https://support.zoom.com/hc/en/article?id=zm_kb&sysparm_article=KB0058296), section 'How to transfer a voice engagement', note on Zoom Phone users. Checked 2026-09-30.
[^51]: A Zoom Contact Center agent can transfer a voice engagement to a Zoom Phone user, Zoom Phone call queue or voicemail, as well as to contact center users, queues and flows. Source: [Transferring a Zoom Contact Center voice engagement](https://support.zoom.com/hc/en/article?id=zm_kb&sysparm_article=KB0058296), section 'How to transfer a voice engagement'. Checked 2026-09-30.
[^52]: During ZPLS failover only the BYOC carrier-provided PSTN number is functional; Zoom Native numbers can only be forwarded to BYOC numbers assigned to users in the same site. Source: [Configuring the Zoom Phone Local Survivability service](https://support.zoom.com/hc/en/article?id=zm_kb&sysparm_article=KB0063728), section on call forwarding and BYOC number setup. Checked 2026-09-30.
[^53]: When a Zoom Phone client fails over to ZPLS, active calls are not preserved; users hear a busy tone and must re-establish the call. Source: [Getting started with Zoom Phone Local Survivability service](https://support.zoom.com/hc/en/article?id=zm_kb&sysparm_article=KB0058798), limitations section. Checked 2026-09-30.
[^54]: Zoom Phone Local Survivability is distributed as a Linux-based appliance run on an on-premises VMware ESXi host as a Zoom Node module. Source: [Getting started with Zoom Phone Local Survivability service](https://support.zoom.com/hc/en/article?id=zm_kb&sysparm_article=KB0058798), overview / infrastructure section. Checked 2026-09-30.
[^55]: During ZPLS failover Zoom Phone supports internal extension dialing, PSTN calling through an SBC, transfer, hold/resume, mute, DTMF, call park and ad-hoc conferencing. Source: [Getting started with Zoom Phone Local Survivability service](https://support.zoom.com/hc/en/article?id=zm_kb&sysparm_article=KB0058798), supported features during failover section. Checked 2026-09-30.
[^56]: ZPLS configuration requires a Business, Education or Enterprise account, a Zoom Node subscription, a Zoom Phone subscription, admin privileges and Multiple Sites enabled for Zoom Phone. Source: [Configuring the Zoom Phone Local Survivability service](https://support.zoom.com/hc/en/article?id=zm_kb&sysparm_article=KB0063728), requirements section at top of article. Checked 2026-09-30.