# SIP trunk capacity and concurrent call planning

Canonical: https://warmtransfer.net/knowledge/sip-trunk-capacity-planning

Last verified: 2026-09-24

ITU-T Recommendation E.520 (11/88), which is in force, covers the number of circuits to provide in automatic and semiautomatic operation without overflow facilities[^7]. In standard SIP signaling, a 503 Service Unavailable response indicates the server temporarily cannot process the request due to overload or maintenance, while a 486 Busy Here response indicates the callee is busy and cannot take the call at that time[^14][^13].

## Webex Calling Local Gateway capacity

Webex Calling supports registration-based and certificate-based Local Gateway (LGW) trunks, which differ in scale, authentication, and device compatibility[^54][^41][^40]. A registration-based LGW trunk supports up to 250 concurrent calls per trunk, a limit attributed to TLS and TCP signaling performance[^54][^39]. Cisco's Local Gateway migration documentation also describes the upgraded Webex Calling infrastructure as allowing up to 250 concurrent sessions per local gateway registration[^27]. In Cisco IOS XE configuration, `max-conn` on the Webex Calling dial-peer (`dial-peer voice 100`) restricts concurrent inbound and outbound calls between the LGW and Webex Calling, and for registration trunks it should be set no higher than 250[^38].

A certificate-based LGW trunk supports more than 250 concurrent calls per trunk, supporting up to 2000 concurrent calls over the internet[^52][^32]. When adding a certificate-based trunk to a location in Control Hub, the administrator enters a maximum number of concurrent calls based on the LGW's capacity[^29].

| Trunk type | Concurrent call capacity | Authentication |
|---|---|---|
| Registration-based | Up to 250[^39] | Digest credentials[^41] |
| Certificate-based | More than 250 (up to 2000 over internet)[^52][^32] | Certificate plus FQDN verification[^41] |

Registration-based trunks only support Cisco CUBE devices, whereas certificate-based trunks support SBCs from Cisco, Oracle, Ribbon, AudioCodes, anynode, and Italtel[^40][^34]. Selecting a certificate-based trunk requires domain claims, public DNS A/SRV records, and a CA-signed certificate with Server Authentication EKU[^33]. Automated troubleshooting is supported for registration-based trunks but is listed as not supported for certificate-based trunks[^30].

During provisioning, each Webex Calling LGW is assigned to 2 Webex Media PoP locations for redundancy and high availability[^37]. Migrating a local gateway to the new Webex Calling proxy addresses re-registers it to the cloud, impacts service, should take no more than 10 to 15 minutes, and is recommended during a maintenance window[^36].

### Capacity guidelines and location scaling

Cisco provides capacity guidelines linking concurrent call requirements, user estimates, trunk types, and minimum link quality:
- Up to 250 concurrent calls (about 2,500 users): registration-based trunk, over-the-top (OTT) link quality[^47].
- About 250 to 2000 concurrent calls (about 20,000 users): certificate-based trunk, OTT link quality[^46].
- About 2000 to 6500 concurrent calls (about 65,000 users): certificate-based trunk, Interconnect minimum link quality[^45].

WarmTransfer's reading of the sources is that 2000 concurrent calls serves as the boundary where minimum link quality transitions from OTT to Interconnect while remaining certificate-based, rather than functioning as a per-trunk hard ceiling[^26]. Furthermore, we infer that the user counts in this table correspond to roughly 10 users per concurrent call across tiers (2,500 for 250; 20,000 for 2000; 65,000 for 6500), though the documentation does not state this ratio or underlying traffic assumptions[^44].

A location using an LGW has a default scale limit of 250 concurrent calls, affecting how locations are sized and split; locations using Cloud Connected PSTN do not have this limit[^53]. WarmTransfer's reading of the sources is that migrating many users into a single location served by 1 registration-based LGW can hit the 250 concurrent call ceiling at peak, where Cisco states the limit is enforced and calls are rejected by Webex Calling access SBCs rather than the carrier[^1]. 

For Webex Contact Center, deployments requiring more than 250 concurrent calls through an LGW must use certificate-based trunks[^51]. The concurrent-call count evaluated against this 250 threshold includes both IVR and agent calls[^50].

In Webex Calling architecture, a location allows up to 100 trunks[^48]. A route group can contain up to 10 trunks, and an enterprise tenant can have up to 10,000 route groups[^49]. High-capacity LGW deployments on Cisco IOS XE may require a High Security (HSEC) license and additional throughput entitlement, though no numeric threshold is specified[^35].

## Carrier and cloud PBX concurrency models

### Twilio Elastic SIP Trunking

Twilio Elastic SIP Trunking accounts provide unlimited concurrent calls by default, dependent on carrier support[^25]. An account can contain up to 100 unique SIP trunks[^15]. However, new accounts lacking an approved Business Profile face limited concurrent calls and cannot modify calls per second (CPS) in the Console; the exact concurrent limit is not stated[^21].

Twilio supports unlimited origination CPS (inbound from the PSTN)[^22]. For trunk termination (outbound to the PSTN), Twilio defaults to 1 CPS per trunk per region[^24]. Raising Twilio termination CPS per region above the default carries a monthly fee, prorated when changed mid-month[^18]. Trunks created outside the Ashburn, VA region must use parent-account pooled CPS, billed per parent account per region[^23]. Exceeding trunk termination CPS generates debugger error 32001 (SIP: Trunk CPS limit exceeded), while exceeding parent-account pooled CPS generates error 32012[^17].

WarmTransfer's reading of the sources is that Asterisk PBX deployments using Twilio Elastic SIP Trunking are constrained by outbound termination CPS and PBX host capacity rather than carrier concurrent call limits, except when operating on new accounts without an approved Business Profile[^16].

### Amazon Connect service quotas

Amazon Connect applies a default quota of 10 concurrent active calls per instance, covering both PSTN and WebRTC calls, which can be raised at the instance level[^3]. Amazon Connect counts calls in a flow, calls waiting in a queue, calls handled by an agent, and outbound calls against this quota; external transfers and callbacks waiting in a callback queue are not counted[^6]. Calls exceeding this quota receive a reorder (fast busy) tone[^4]. AWS states smaller quota increases can be approved within hours, larger increases take up to 3 weeks, and extra-large worldwide increases may take months[^5]. Additionally, the concurrent campaign active calls quota defaults to 0 and must be increased before outbound campaigns place calls; exceeding it results in a fast busy tone[^2].

### Microsoft Teams Direct Routing

In Microsoft Teams Direct Routing, the `MaxConcurrentSessions` parameter on an SBC gateway is used by the alerting system, notifying the administrator when concurrent sessions reach 90% or more of the configured value[^9]. If left unset, concurrency alerts are disabled, though concurrent session metrics continue to be reported every 24 hours[^11].

Direct Routing balances outbound traffic across healthy SBCs of equal priority in a route using a Fisher-Yates shuffle, determining health based on SIP OPTIONS messages received in the prior 3 minutes[^12]. If an outbound INVITE receives a 408, 503, or 504 response, Direct Routing fails over to another SBC in the voice routing policy by default; other 4xx and 6xx responses terminate the call unless `FailoverResponseCodes` is modified[^8].

## See also

- [Local Gateway design and configuration](https://warmtransfer.net/knowledge/webex-calling-local-gateway)
- [Setting up an Asterisk PBX with Twilio Elastic SIP Trunking](https://warmtransfer.net/knowledge/asterisk-twilio-pbx-setup)
- [Trunks route groups and dial plans in Control Hub](https://warmtransfer.net/knowledge/webex-calling-trunks-route-groups)
- [Cisco Unified Border Element configuration and licensing](https://warmtransfer.net/knowledge/cube-configuration)
- [Remote site survivability patterns](https://warmtransfer.net/knowledge/survivability-remote-site)
- [Session border controller selection and certification](https://warmtransfer.net/knowledge/sbc-selection)
- [Enterprise dial plan design and normalization](https://warmtransfer.net/knowledge/dial-plan-design)
- [Unified CM SIP trunk to CUBE configuration](https://warmtransfer.net/knowledge/cucm-config-sip-trunk-to-cube)

## Applicability

Applies to: ITU-T E-series traffic engineering, IETF SIP, Cisco Webex Calling, Cisco Webex Calling Local Gateway, Cisco IOS XE CUBE as Webex Calling Local Gateway, Cisco Webex Contact Center, Twilio Elastic SIP Trunking, Amazon Web Services Amazon Connect, and Microsoft Teams Phone Direct Routing. Deployments: multi-tenant, on-premises-gateway, and standard. Sources checked 2026-09-24. On Cisco IOS XE, a Webex Calling registration-based Local Gateway requires release 17.6.1a or later[^42]. A certificate-based Local Gateway on Cisco IOS XE requires release 17.9.1a or later[^31].

## What remains uncertain

Per-platform CUBE session capacity as Webex Calling LGW is not covered by the sources below. The specific SIP response Webex Calling access SBCs send when a registering LGW exceeds 250 calls is not covered by the sources below. Whether several registration trunks on one CUBE or in a route group legitimately exceed 250 calls is not covered by the sources below. Whether Webex enforces any per-trunk cap on certificate-based trunks (6500 or the Control Hub max concurrent field) is not covered by the sources below. HSEC license and throughput entitlement thresholds for high-capacity LGW are not covered by the sources below. Per-call bandwidth by codec for WAN sizing of SIP trunks is not covered by the sources below. Vendor or standards method for busy-hour traffic to channel count (Erlang B) with worked values is not covered by the sources below. Whether Teams Direct Routing enforces any per-SBC concurrent session limit is not covered by the sources below.

## Sources

[^1]: A migration that concentrates many users into a single Webex Calling location served by 1 registration-based Local Gateway can reach the documented 250 concurrent call ceiling at peak, at which point Cisco states the limit is enforced and additional calls are rejected by the Webex Calling access session border controllers rather than by the carrier (inferred). Source: [Cisco Preferred Architecture for Webex Calling — Design Overview](https://www.cisco.com/c/dam/en/us/td/docs/solutions/CVD/Collaboration/hybrid/AltDesigns/PA-WbxCall.pdf), Registration-based Local Gateway page 23, Table 2 page 26 and Location Definitions pages 60-61. Checked 2026-09-16.
[^2]: Amazon Connect's 'Concurrent campaign active calls per instance' quota defaults to 0 and must be raised before outbound campaigns can place calls; exceeding it gives contacts a fast busy tone. Source: [Connect Customer service quotas](https://docs.aws.amazon.com/connect/latest/adminguide/amazon-connect-service-limits.html), Section 'Connect Customer Outbound campaigns service quotas', row 'Concurrent campaign active calls per instance'. Checked 2026-09-24.
[^3]: Amazon Connect's 'Concurrent active calls per instance' quota defaults to 10, covers PSTN and WebRTC calls, and can be raised at the resource (instance) level. Source: [Connect Customer service quotas](https://docs.aws.amazon.com/connect/latest/adminguide/amazon-connect-service-limits.html), Section 'Connect Customer quotas', row 'Concurrent active calls per instance'. Checked 2026-09-24.
[^4]: When the Amazon Connect concurrent active calls quota is exceeded, contacts get a reorder (fast busy) tone. Source: [Connect Customer service quotas](https://docs.aws.amazon.com/connect/latest/adminguide/amazon-connect-service-limits.html), Section 'How contacts are counted'. Checked 2026-09-24.
[^5]: AWS says smaller Amazon Connect quota increases can be approved in hours, larger ones can take up to 3 weeks, and an extra-large worldwide increase can take months. Source: [Connect Customer service quotas](https://docs.aws.amazon.com/connect/latest/adminguide/amazon-connect-service-limits.html), Section 'Important things to know'. Checked 2026-09-24.
[^6]: Amazon Connect counts calls in a flow, waiting in queue, handled by an agent, and outbound calls against the concurrent-calls quota, but not callbacks waiting in a callback queue or external transfers. Source: [Connect Customer service quotas](https://docs.aws.amazon.com/connect/latest/adminguide/amazon-connect-service-limits.html), Section 'How contacts are counted'. Checked 2026-09-24.
[^7]: ITU-T Recommendation E.520 (11/88), which is in force, covers the number of circuits to provide in automatic and semiautomatic operation without overflow facilities. Source: [E.520 : Number of circuits to be provided in automatic and/or semiautomatic operation, without overflow facilities](https://www.itu.int/rec/T-REC-E.520), Recommendation landing page: title, status 'In force', approval 1988-11-25. Checked 2026-09-24.
[^8]: By default Direct Routing tries another SBC in the user's voice routing policy when an outbound INVITE gets 408, 503 or 504; other 4xx and 6xx responses end the call unless FailoverResponseCodes is changed. Source: [New-CsOnlinePSTNGateway (MicrosoftTeams)](https://learn.microsoft.com/en-us/powershell/module/microsoftteams/new-csonlinepstngateway), Parameters, '-FailoverResponseCodes' (default value 408, 503, 504). Checked 2026-09-24.
[^9]: In Teams Direct Routing, the MaxConcurrentSessions setting on an SBC (PSTN gateway) is used by the alerting system, which alerts the tenant administrator when concurrent sessions reach 90% or more of the configured value. Source: [New-CsOnlinePSTNGateway (MicrosoftTeams)](https://learn.microsoft.com/en-us/powershell/module/microsoftteams/new-csonlinepstngateway), Parameters, '-MaxConcurrentSessions'. Checked 2026-09-24.
[^10]: The parameter reference describes MaxConcurrentSessions only as an alerting threshold and says nothing about calls being rejected above it, so it should not be treated as an enforced cap, even though the cmdlet summary calls it 'the limit of maximum concurrent sessions' (inferred). Source: [New-CsOnlinePSTNGateway (MicrosoftTeams)](https://learn.microsoft.com/en-us/powershell/module/microsoftteams/new-csonlinepstngateway), Description section versus Parameters '-MaxConcurrentSessions'. Checked 2026-09-24.
[^11]: If MaxConcurrentSessions is not set on a Direct Routing SBC, no concurrency alerts are generated, though the monitoring system still reports the number of concurrent sessions every 24 hours. Source: [New-CsOnlinePSTNGateway (MicrosoftTeams)](https://learn.microsoft.com/en-us/powershell/module/microsoftteams/new-csonlinepstngateway), Parameters, '-MaxConcurrentSessions'. Checked 2026-09-24.
[^12]: When two or more SBCs in a Direct Routing route are healthy and of equal priority, Direct Routing distributes calls between them with a Fisher-Yates shuffle; SBC health is judged from SIP OPTIONS received in the last three minutes. Source: [Monitor Direct Routing](https://learn.microsoft.com/en-us/microsoftteams/direct-routing-monitor-and-troubleshoot), Section 'Monitoring availability of Session Border Controllers using Session Initiation Protocol (SIP) options messages'. Checked 2026-09-24.
[^13]: In SIP, 486 Busy Here means the callee is busy and cannot take the call now, but the caller may try again later. Source: [RFC 3261 — SIP: Session Initiation Protocol](https://www.rfc-editor.org/rfc/rfc3261.html), Section 21.4.24 '486 Busy Here'. Checked 2026-09-24.
[^14]: In SIP, a 503 Service Unavailable response means the server temporarily cannot process the request because of overload or maintenance. Source: [RFC 3261 — SIP: Session Initiation Protocol](https://www.rfc-editor.org/rfc/rfc3261.html), Section 21.5.4 '503 Service Unavailable'. Checked 2026-09-24.
[^15]: A Twilio account can have up to 100 unique SIP trunks. Source: [Twilio SIP Trunking Scale and Limits](https://www.twilio.com/docs/sip-trunking/scale-and-limits), Section 'Accounts'. Checked 2026-09-24.
[^16]: For an Asterisk PBX on Twilio Elastic SIP Trunking, the trunk-side constraint to plan around is outbound termination CPS and PBX host capacity, not a provider concurrent-call ceiling, except on new accounts without an approved Business Profile (inferred). Source: [Twilio SIP Trunking Scale and Limits](https://www.twilio.com/docs/sip-trunking/scale-and-limits), Section 'Accounts' read with twilio-docs-cps-trunk-termination default CPS passage. Checked 2026-09-24.
[^17]: When Twilio trunk termination CPS is exceeded, Twilio raises debugger error 32001 (SIP: Trunk CPS limit exceeded); exceeding parent-account pooled CPS raises error 32012. Source: [Calls per Second (CPS) — Trunking Termination](https://www.twilio.com/docs/sip-trunking/cps-trunk-termination), CPS exceeded / debugger alerts passage. Checked 2026-09-24.
[^18]: Raising Twilio termination CPS per region above the default carries a monthly fee, prorated when changed mid-month. Source: [Calls per Second (CPS) — Trunking Termination](https://www.twilio.com/docs/sip-trunking/cps-trunk-termination), CPS pricing passage. Checked 2026-09-24.
[^19]: Twilio's October 2020 changelog announced that customers could set Elastic SIP Trunk CPS between 1 and 15 per region (disputed). Source: [Upgrade and Downgrade your Elastic SIP Trunking CPS (Calls per Second) on demand](https://www.twilio.com/en-us/changelog/upgrade-and-downgrade-your-elastic-sip-trunking-cps--calls-per-s), Changelog entry body. Checked 2026-09-24.
[^20]: Twilio's current termination CPS documentation lets customers set per-trunk CPS between 1 and 5, and directs requests above 5 to the sales team (disputed). Source: [Calls per Second (CPS) — Trunking Termination](https://www.twilio.com/docs/sip-trunking/cps-trunk-termination), Trunk-level CPS range passage. Checked 2026-09-24.
[^21]: New Twilio accounts without an approved Business Profile have limited concurrent calls and cannot change CPS themselves in Console; the limit value is not stated. Source: [Twilio SIP Trunking Scale and Limits](https://www.twilio.com/docs/sip-trunking/scale-and-limits), Section 'Accounts', caveat beneath the limits list. Checked 2026-09-24.
[^22]: Twilio Elastic SIP Trunking allows unlimited origination (inbound from PSTN) calls per second. Source: [Twilio SIP Trunking Scale and Limits](https://www.twilio.com/docs/sip-trunking/scale-and-limits), Section 'Accounts'. Checked 2026-09-24.
[^23]: Twilio trunks created in a region other than Ashburn, VA must use parent-account pooled CPS, billed per parent account per region. Source: [Calls per Second (CPS) — Trunking Termination](https://www.twilio.com/docs/sip-trunking/cps-trunk-termination), Pooled (parent account) CPS passage. Checked 2026-09-24.
[^24]: By default Twilio Elastic SIP Trunking allows 1 termination (outbound to PSTN) call per second per trunk per region. Source: [Calls per Second (CPS) — Trunking Termination](https://www.twilio.com/docs/sip-trunking/cps-trunk-termination), Trunk-level CPS default passage. Checked 2026-09-24.
[^25]: Twilio Elastic SIP Trunking accounts have unlimited concurrent calls by default, dependent on carrier support. Source: [Twilio SIP Trunking Scale and Limits](https://www.twilio.com/docs/sip-trunking/scale-and-limits), Section 'Accounts'. Checked 2026-09-24.
[^26]: In Cisco's Local Gateway guideline, 2000 concurrent calls is the boundary at which the minimum link quality changes from OTT to Interconnect while the trunk type stays certificate-based; the guideline does not present 2000 as a per-trunk hard ceiling (inferred). Source: [Get started with Local Gateway](https://help.webex.com/en-us/article/t9xctu/Get-started-with-Local-Gateway), Calling capacity guideline table, rows '~ 250–2000' and '~ 2000–6500' read together. Checked 2026-09-24.
[^27]: Cisco's Local Gateway migration article describes the upgraded Webex Calling infrastructure as allowing up to 250 concurrent sessions per local gateway registration. Source: [Migrate your local gateway](https://help.webex.com/en-us/article/n8xqiwr/Migrate-your-local-gateway), Benefits passage near the start of 'Migrate your local gateway'. Checked 2026-09-24.
[^28]: Cisco's Local Gateway guide lists a diagnostic signature (DS 65095) that detects the IOS XE CCAPI 'Call spike threshold' internal-error syslog; the guide gives no spike threshold value. Source: [Configure Local Gateway on Cisco IOS XE for Webex Calling](https://help.webex.com/en-us/article/jr1i3r/Configure-Local-Gateway-on-Cisco-IOS-XE-for-Webex-Calling), Diagnostic signatures section, entry DS 65095. Checked 2026-09-24.
[^29]: When adding a certificate-based trunk to a location in Control Hub, the administrator enters a maximum number of concurrent calls, which must be set according to the Local Gateway's capacity. Source: [Configure trunks, route groups, and dial plans for Webex Calling](https://help.webex.com/article/n0xb944/Configure-Trunks-Route-Groups-and-Dial-Plans-for-Cisco-Webex-Calling), Section 'Add a certificate-based trunk to a location'. Checked 2026-09-24.
[^30]: Automated troubleshooting is listed as supported for registration-based Local Gateway trunks and not supported for certificate-based trunks. Source: [Get started with Local Gateway](https://help.webex.com/en-us/article/t9xctu/Get-started-with-Local-Gateway), Local Gateway trunking models table, row 'Automated troubleshooting'. Checked 2026-09-24.
[^31]: A Webex Calling certificate-based Local Gateway on Cisco IOS XE requires release 17.9.1a or later. Source: [Configure Local Gateway on Cisco IOS XE for Webex Calling](https://help.webex.com/en-us/article/jr1i3r/Configure-Local-Gateway-on-Cisco-IOS-XE-for-Webex-Calling), Section 'Configure your Local Gateway', trunk-type prerequisites. Checked 2026-09-24.
[^32]: A Webex Calling certificate-based Local Gateway trunk supports more than 250 concurrent calls per trunk. Source: [Get started with Local Gateway](https://help.webex.com/en-us/article/t9xctu/Get-started-with-Local-Gateway), Local Gateway trunking models table, row 'Concurrent calls', column 'Certificate-based'. Checked 2026-09-24.
[^33]: Choosing a certificate-based Webex Calling trunk for capacity above 250 calls also requires domain claims, public DNS A/SRV records, and a CA-signed certificate with the Server Authentication EKU. Source: [Get started with Local Gateway](https://help.webex.com/en-us/article/t9xctu/Get-started-with-Local-Gateway), Local Gateway trunking models table, rows 'DNS requirements' and 'Certificate needs'. Checked 2026-09-24.
[^34]: A Webex Calling certificate-based Local Gateway trunk supports SBCs from multiple vendors, listed as Cisco, Oracle, Ribbon, AudioCodes, anynode and Italtel. Source: [Get started with Local Gateway](https://help.webex.com/en-us/article/t9xctu/Get-started-with-Local-Gateway), Local Gateway trunking models table, row 'Device type', column 'Certificate-based'. Checked 2026-09-24.
[^35]: For high-capacity Local Gateway deployments on Cisco IOS XE, a High Security (HSEC) license and additional throughput entitlement may be required; the article gives no numeric threshold. Source: [Configure Local Gateway on Cisco IOS XE for Webex Calling](https://help.webex.com/en-us/article/jr1i3r/Configure-Local-Gateway-on-Cisco-IOS-XE-for-Webex-Calling), Section 'Configure your Local Gateway', licensing prerequisites. Checked 2026-09-24.
[^36]: Migrating a local gateway to the new Webex Calling proxy addresses re-registers it to the cloud, impacts service, should take no more than 10 to 15 minutes, and is recommended during a maintenance window. Source: [Migrate your local gateway](https://help.webex.com/en-us/article/n8xqiwr/Migrate-your-local-gateway), Migrate your local gateway, service impact and duration passages. Checked 2026-09-24.
[^37]: During provisioning each Webex Calling Local Gateway is assigned to two Webex Media PoP locations for redundancy and high availability. Source: [Get started with Local Gateway](https://help.webex.com/en-us/article/t9xctu/Get-started-with-Local-Gateway), Get started with Local Gateway, provisioning/redundancy passage. Checked 2026-09-24.
[^38]: In Cisco's IOS XE Local Gateway configuration, max-conn on the Webex Calling dial-peer (dial-peer voice 100) restricts concurrent inbound and outbound calls between the LGW and Webex Calling, and for registration trunks it should be set no higher than 250. Source: [Configure Local Gateway on Cisco IOS XE for Webex Calling](https://help.webex.com/en-us/article/jr1i3r/Configure-Local-Gateway-on-Cisco-IOS-XE-for-Webex-Calling), Section 'Configure Webex Calling registration-based trunk', dial-peer voice 100 voip block and its explanation of max-conn 250. Checked 2026-09-24.
[^39]: A Webex Calling registration-based Local Gateway trunk supports up to 250 concurrent calls per trunk. Source: [Get started with Local Gateway](https://help.webex.com/en-us/article/t9xctu/Get-started-with-Local-Gateway), Local Gateway trunking models table, row 'Concurrent calls', column 'Registration-based'. Checked 2026-09-24.
[^40]: A Webex Calling registration-based Local Gateway trunk can only use Cisco CUBE as the device. Source: [Get started with Local Gateway](https://help.webex.com/en-us/article/t9xctu/Get-started-with-Local-Gateway), Local Gateway trunking models table, row 'Device type'. Checked 2026-09-24.
[^41]: A Webex Calling registration-based trunk authenticates with digest credentials (username and password), while a certificate-based trunk authenticates with a certificate plus FQDN verification. Source: [Get started with Local Gateway](https://help.webex.com/en-us/article/t9xctu/Get-started-with-Local-Gateway), Local Gateway trunking models table, row 'Authentication'. Checked 2026-09-24.
[^42]: A Webex Calling registration-based Local Gateway on Cisco IOS XE requires release 17.6.1a or later. Source: [Configure Local Gateway on Cisco IOS XE for Webex Calling](https://help.webex.com/en-us/article/jr1i3r/Configure-Local-Gateway-on-Cisco-IOS-XE-for-Webex-Calling), Section 'Configure your Local Gateway', trunk-type prerequisites. Checked 2026-09-24.
[^43]: Grouping several registration-based trunks in a route group could in principle raise aggregate concurrency beyond 250 calls, but no Cisco page read for this packet documents route groups as a capacity mechanism or how calls are distributed across member trunks (inferred). Source: [Configure trunks, route groups, and dial plans for Webex Calling](https://help.webex.com/article/n0xb944/Configure-Trunks-Route-Groups-and-Dial-Plans-for-Cisco-Webex-Calling), Section 'Configurable limitations' (trunks per route group) combined with the per-trunk 250 limit in cisco-help-t9xctu-get-started-local-gateway. Checked 2026-09-24.
[^44]: The user counts in Cisco's Local Gateway capacity table correspond to roughly ten users per concurrent call at every tier (2,500 for 250; 20,000 for 2000; 65,000 for 6500), but the page does not state this ratio or its traffic assumptions (inferred). Source: [Get started with Local Gateway](https://help.webex.com/en-us/article/t9xctu/Get-started-with-Local-Gateway), Calling capacity guideline table, 'Approximate user numbers' column divided by upper bound of 'Concurrent call requirement'. Checked 2026-09-24.
[^45]: Cisco's Local Gateway capacity guideline maps a requirement of about 2000 to 6500 concurrent calls (about 65,000 users) to a certificate-based trunk, with Interconnect as the minimum link quality. Source: [Get started with Local Gateway](https://help.webex.com/en-us/article/t9xctu/Get-started-with-Local-Gateway), Calling capacity guideline table, row '~ 2000–6500'. Checked 2026-09-24.
[^46]: Cisco's Local Gateway capacity guideline maps a requirement of about 250 to 2000 concurrent calls (about 20,000 users) to a certificate-based trunk, with over-the-top (OTT) as the minimum link quality. Source: [Get started with Local Gateway](https://help.webex.com/en-us/article/t9xctu/Get-started-with-Local-Gateway), Calling capacity guideline table, row '~ 250–2000'. Checked 2026-09-24.
[^47]: Cisco's Local Gateway capacity guideline maps a requirement of up to 250 concurrent calls (about 2,500 users) to a registration-based trunk, with over-the-top (OTT) as the minimum link quality. Source: [Get started with Local Gateway](https://help.webex.com/en-us/article/t9xctu/Get-started-with-Local-Gateway), Calling capacity guideline table (columns: Concurrent call requirement / Approximate user numbers / Trunk type Preference / Minimum link quality), row 'Up to 250'. Checked 2026-09-24.
[^48]: Webex Calling allows up to 100 trunks per location. Source: [Configure trunks, route groups, and dial plans for Webex Calling](https://help.webex.com/article/n0xb944/Configure-Trunks-Route-Groups-and-Dial-Plans-for-Cisco-Webex-Calling), Section 'Configurable limitations', row 'Trunks per location'. Checked 2026-09-24.
[^49]: A Webex Calling route group can contain up to 10 trunks, and an enterprise can have up to 10,000 route groups. Source: [Configure trunks, route groups, and dial plans for Webex Calling](https://help.webex.com/article/n0xb944/Configure-Trunks-Route-Groups-and-Dial-Plans-for-Cisco-Webex-Calling), Section 'Configurable limitations', rows 'Trunks per route group' and 'Route groups per enterprise'. Checked 2026-09-24.
[^50]: For Webex Contact Center, the concurrent-call count compared against the 250 threshold includes both IVR and agent calls. Source: [Set up voice channels for Webex Contact Center](https://help.webex.com/en-us/article/2dputx/Set-up-voice-channel-for-Webex-Contact-Center), Section 'Configuration of Webex Local Gateway', parenthetical '(IVR + Agent concurrent calls)'. Checked 2026-09-24.
[^51]: Webex Contact Center deployments that need more than 250 concurrent calls through a Local Gateway must use the certificate-based trunk type. Source: [Set up voice channels for Webex Contact Center](https://help.webex.com/en-us/article/2dputx/Set-up-voice-channel-for-Webex-Contact-Center), Section 'Configuration of Webex Local Gateway'. Checked 2026-09-24.
[^52]: A certificate-based Local Gateway trunk supports up to 2000 concurrent calls over the internet, an order of magnitude more than the registration-based model. Source: [Cisco Preferred Architecture for Webex Calling — Design Overview](https://www.cisco.com/c/dam/en/us/td/docs/solutions/CVD/Collaboration/hybrid/AltDesigns/PA-WbxCall.pdf), Table 2, Certificate versus registration based Local Gateway, page 27. Checked 2026-09-04.
[^53]: A location using a Local Gateway is subject to a default scale limit of 250 concurrent calls, which affects how locations should be sized and split; locations using Cloud Connected PSTN are not subject to that limit. Source: [Cisco Preferred Architecture for Webex Calling — Design Overview](https://www.cisco.com/c/dam/en/us/td/docs/solutions/CVD/Collaboration/hybrid/AltDesigns/PA-WbxCall.pdf), Location Definitions, Deployment Aspects, pages 60-61. Checked 2026-09-04.
[^54]: A registration-based Local Gateway is limited to 250 concurrent calls per registering gateway, a ceiling attributed to TLS and TCP signalling performance. Source: [Cisco Preferred Architecture for Webex Calling — Design Overview](https://www.cisco.com/c/dam/en/us/td/docs/solutions/CVD/Collaboration/hybrid/AltDesigns/PA-WbxCall.pdf), Registration-based Local Gateway, page 23. Checked 2026-09-04.