WarmTransfer
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SRST on Cisco IOS XE

Verified 2026-09-25 · 60 sources · tier 2–6

Also known as Cisco Unified SRST, E-SRST, Enhanced SRST, SCCP SRST, SIP SRST, Survivable Remote Site Telephony.

Cisco Unified SRST provides fallback call control for Cisco IP phones at a remote site when their connection to primary call control (Unified CM) is lost, whether from a WAN outage or a Unified CM failure 13. For SIP phones, Unified SIP SRST backs up an external SIP call control (IP-PBX) by providing basic registrar and redirect server or back-to-back user agent (B2BUA) services 49.

How it works

In Unified CM, an SRST reference is assigned per device pool, and every phone in that pool tries to connect to the assigned SRST gateway when it cannot reach any Unified CM node 11. The device pool's SRST reference can be Disable (no fallback attempt), Use Default Gateway (the phone's IP default gateway acts as SRST), or a user-defined SRST reference that names a specific gateway 10. Unified CM will not delete an SRST reference that is still assigned to a device pool, and a deleted SRST reference cannot be recovered 9.

When the connection to Unified CM is restored, the IP phones automatically cancel their registration with the SRST router and re-register with Unified CM 19. Connection Monitor Duration controls how long a phone monitors its restored connection to Unified CM before leaving SRST 8. It defaults to 120 seconds (maximum 2,592,000) and is set as an enterprise parameter or per device pool to stop phones flapping between SRST and Unified CM 8. A phone engaged in an active call cannot re-establish its connection to the primary Unified CM until that call ends 2. WarmTransfer's reading of the sources is that with default settings, a fallback-return test should expect idle phones to stay on the SRST router for about 2 minutes after the WAN is restored, and phones on active calls to stay until those calls end 20.

For Webex Calling deployments, Unified SRST 14.3 introduced Webex Survivability Gateway mode, which provides survivability for Webex Calling endpoints as well as Unified CM-registered endpoints 40. The Survivability Gateway is enabled with mode webex-sgw under voice register global, and the Webex cloud includes the gateway address, hostname, and port in the device configuration instead of relying on a Unified CM SRST reference 60. Webex Calling registrations and call control revert from the Survivability Gateway to the Webex cloud once the Webex connection has been restored for at least 30 seconds 58.

Configuring SIP SRST and SCCP SRST

SIP SRST is enabled under voice service voip with allow-connections sip to sip and, in the sip sub-mode, the registrar server command with optional expires max and min values 47. SIP SRST capacity is set under voice register global with max-pool (maximum SIP phone pools) and max-dn, and max-pool is platform-dependent 56. A voice register pool uses id network/mask, id ip/mask, or id mac to decide which SIP phone registrations the SRST router accepts, and can carry dial-peer attributes such as dtmf-relay and voice-class codec 57. When a SIP phone registers and matches a voice register pool that has a proxy configured, SRST automatically creates 2 dial peers: one routing to the proxy and a fallback dial peer routing to the phone 15. The mode command under voice register global takes the keywords cme or esrst; no srst keyword is documented 36. WarmTransfer's reading of the sources is that setting mode cme under voice register global makes the router provision SIP phones as a Unified CME system instead of acting as SRST, so it should not be set on a router meant for plain SRST 35.

SCCP SRST is enabled with the global configuration command call-manager-fallback, which enters call-manager-fallback configuration mode; the feature is off until it is configured 7. Under call-manager-fallback, ip source-address sets the address and port (default 2000) on which the router accepts SCCP phone messages, and the strict-match keyword enforces strict IP address verification 22. In call-manager-fallback mode, max-ephones and max-dn both default to 0, and their maximum values are platform-dependent 33. After max-ephones or max-dn has been set, the router must be rebooted to reduce the limit 38.

SCCP and SIP phones can be mixed on one SRST router, provided that the combined total does not exceed the platform's listed maximum 34.

Call handling and fallback features

PSTN access during SRST fallback depends on POTS dial peers whose destination-pattern maps dialled digits to local voice ports (FXO, E&M, BRI or PRI) 37. The access-code command in call-manager-fallback creates temporary POTS dial peers for PSTN trunk access during fallback when the permanent configuration lacks them 1. The dialplan-pattern command expands abbreviated extensions into fully qualified E.164 numbers during fallback, and up to 5 patterns can be configured 14. The alias command in call-manager-fallback reroutes calls for numbers that are not registered to an alternate number, which must belong to a registered phone; up to 50 alias rules are allowed 3. During fallback, call-forward busy and call-forward noan (with a ring timeout) forward calls to a set directory number, for example a voicemail pilot 6.

Classic Unified SIP SRST does not support BLF speed-dial notification or call-forward synchronization, and music on hold is not supported for calls that SIP phones put on hold 45. The Unified SRST restrictions state that all calls must use the same codec, either G.711 or G.729r8 44. On the ISR 4000, Unified SIP SRST does not support transcoding, shared lines or mixed shared lines, video calling, or multicast MOH for SIP 48.

Enhanced SRST (E-SRST)

Enhanced SRST is enabled with mode esrst under telephony-service for SCCP phones and under voice register global for SIP phones 18. E-SRST adds features beyond classic SRST, including advanced voice hunt groups (HLog, statistics, mixed SIP/SCCP members), shared lines, BLF monitoring, video calls, and basic ACD (B-ACD) 16. Cisco Unified SRST Manager is end-of-life, and E-SRST is now provisioned only manually through the CLI 54.

Licensing

From Unified SRST 12.5 (IOS XE 16.10.1a), paper Right-To-Use licenses are no longer supported and Smart Licensing entitlements are mandatory 52. Smart Licensing Using Policy was introduced for Unified SRST in release 14.1 (IOS XE 17.3.2 through 17.6.1a timeframe) 51. Cisco SRST can be bought as part of a Cisco Collaboration Flex subscription, where the entitlements come with on-premises calling licenses 53.

Unified SRST requires 1 SRST_EP license entitlement for each configured SIP or SCCP phone 24. Unified E-SRST requires 1 SRST_E_EP license entitlement for each configured SIP or SCCP phone 17. SRST license usage is reported 3 minutes after the last configuration change 23. Starting with Cisco IOS XE 17.18.1a, Smart Licensing reports SRST usage with the flex subscription entitlement tag SRST_EP_SUB across all supported platforms 25. The SRST feature roadmap for release 15.0 says a Collaboration Flex subscription is required for ongoing support, alongside the switch to reporting a flex subscription entitlement tag 21.

Platform capacity and release support

Unified SIP SRST on the ISR 4000 was introduced in Unified SRST 10.0 (IOS XE 3.10S) and extended to all ISR 4000 models listed in the chapter in Unified SRST 10.5 (IOS XE 3.13S) 46. Unified SRST/E-SRST 14.3 is supported only on platforms running Cisco IOS XE Cupertino 17.9.1a and later 39. Unified SRST 14.4 is included in Cisco IOS XE 17.14.1a and later and added TLS 1.3 support for secure SIP and SCCP SRST 41. The TLS 1.3 support added in SRST 14.4 applies to SCCP analog voice gateways but not to SCCP IP phone endpoints 55. The Webex Calling Survivability Gateway requires Cisco IOS XE Dublin 17.12.3 or later and must use an IPv4 address, because IPv6 is not supported 59. The C8375-E-G2 is supported for E-SRST from Cisco IOS XE 17.18.2, and is listed in SRST 15.0 with virtual DSP enabled 5.

Platform Phone capacity Line capacity
ISR 1100 50 28 200 28
ISR 4321 50 28 200 28
ISR 4331 100 29 400 29
ISR 4351 700 29 1,000 29
ISR 4431 1,200 30 1,800 30
C8200L-1N-4T 1,500 27 2,500 27
C8200-1N-4T 2,500 27 3,500 27
C8300 (all 4 models) 2,500 27 3,500 27
Catalyst 8000V Small 500 26 2,000 26
Catalyst 8000V Medium 1,000 26 2,000 26
Catalyst 8000V Large 2,000 26 2,000 26

The Catalyst 8000V does not support FXO or T1/E1 PSTN trunk configurations for SRST 4.

Diagnosing it

The documented SIP SRST verification commands include show sip-ua status registrar, show voice register dial-peers, show dial-peer voice, and show license summary 50. SCCP SRST is verified with show call-manager-fallback all (feature enabled) and show ephone (phones registered to the SRST router) 43.

See also

Applicability

Applies to: Cisco Unified SRST, Cisco Unified E-SRST, Cisco Unified Communications Manager, and Cisco Webex Calling Survivability Gateway. Deployments: cloud and on-premises. Sources checked 2026-09-25. Unified SRST 10.0 applies to Cisco IOS XE 3.10S, and Unified SRST 10.5 applies to IOS XE 3.13S 46. Unified SRST 12.5 applies to IOS XE 16.10.1a and later 52. Unified SRST 14.1 applies across the IOS XE 17.3.2 through 17.6.1a timeframe 51. Unified SRST/E-SRST 14.3 applies to Cisco IOS XE Cupertino 17.9.1a through 17.11.1a and later 39​40. Webex Calling Survivability Gateway applies to Cisco IOS XE Dublin 17.12.3 and later 59. Unified SRST 14.4 applies to Cisco IOS XE 17.14.1a and later 41. Unified SRST 15.0 applies to Cisco IOS XE 17.18.1a and later 21. The C8375-E-G2 platform applies to Cisco IOS XE 17.18.2 and later 5.

What remains uncertain

What happens to perpetual SRST_EP entitlements after the move to SRST_EP_SUB and Flex is not covered by the sources below. Outbound PSTN over a SIP trunk during SRST (CUBE colocation and dial-peer design) is not covered by the sources below. Emergency calling behaviour and ELIN handling during SRST fallback are not covered by the sources below. SRST and E-SRST phone capacity on the C8375-E-G2 is not covered by the sources below. The capacity and memory table for SRST 14.4 and 15.0 is not covered by the sources below. Any vendor procedure for testing SRST fallback and fallback return is not covered by the sources below. SIP phone fallback detection timing (registration expiry and keepalive behaviour to Unified CM) is not covered by the sources below. Any vendor statement of the default voice register global mode (SRST when no mode is set) is not covered by the sources below.

See also

Related to

Referenced by

  • Unity Connection networking — The Release 15 design guide networking chapter covers Survivable Remote Site Voicemail alongside SRST; its interaction with HTTPS networking is not documented in the sources read.

Sources

  1. 1
    The access-code command in call-manager-fallback creates temporary POTS dial peers for PSTN trunk access during fallback when the permanent configuration lacks them.
    Cisco Unified SRST Administration Guide (All Versions) - Configuring Call Handling · Configuring Call Handling > Trunk Access Codes · Checked 2026-09-25
  2. 2
    A phone engaged in an active call cannot re-establish its connection to the primary Unified CM until that call ends.
    Cisco Unified SRST Administration Guide (All Versions) - Cisco Unified SRST Feature Overview · Cisco Unified SRST Feature Overview > Cisco Unified IP Phone Fallback (restoration) · Checked 2026-09-25
  3. 3
    The alias command in call-manager-fallback reroutes calls for numbers that are not registered to an alternate number, which must belong to a registered phone; up to 50 alias rules are allowed.
    Cisco Unified SRST Administration Guide (All Versions) - Configuring Call Handling · Configuring Call Handling > Call Rerouting (alias) · Checked 2026-09-25
  4. 4
    The Catalyst 8000V does not support FXO or T1/E1 PSTN trunk configurations for SRST.
    Unified SRST/E-SRST 14.3 Supported Firmware, Platforms, Memory, and Voice Products · Platform notes > Catalyst 8000V · Checked 2026-09-25
  5. 5
    The C8375-E-G2 is supported for E-SRST from Cisco IOS XE 17.18.2, and is listed in SRST 15.0 with virtual DSP enabled.
    Cisco Unified SRST Administration Guide (All Versions) - Enhanced SRST · Enhanced SRST > supported platforms · Checked 2026-09-25
  6. 6
    During fallback, call-forward busy and call-forward noan (with a ring timeout) forward calls to a set directory number, for example a voicemail pilot.
    Cisco Unified SRST Administration Guide (All Versions) - Configuring Call Handling · Configuring Call Handling > Call Forwarding / Voicemail Integration · Checked 2026-09-25
  7. 7
    SCCP SRST is enabled with the global configuration command call-manager-fallback, which enters call-manager-fallback configuration mode; the feature is off until it is configured.
    Cisco Unified SRST and Cisco Unified SIP SRST Command Reference (All Versions) - Command Reference: A through M · Command Reference A through M > call-manager-fallback · Checked 2026-09-25
  8. 8
    Connection Monitor Duration controls how long a phone monitors its restored connection to Unified CM before leaving SRST. It defaults to 120 seconds (maximum 2,592,000) and is set as an enterprise parameter or per device pool to stop phones flapping between SRST and Unified CM.
    System Configuration Guide for Cisco Unified Communications Manager, Release 15 and SUs - Configure SRST · Configure SRST > Connection Monitor Duration · Checked 2026-09-25
  9. 9
    Unified CM will not delete an SRST reference that is still assigned to a device pool, and a deleted SRST reference cannot be recovered.
    System Configuration Guide for Cisco Unified Communications Manager, Release 15 and SUs - Configure SRST · Configure SRST > SRST Restrictions · Checked 2026-09-25
  10. 10
    The device pool's SRST reference can be Disable (no fallback attempt), Use Default Gateway (the phone's IP default gateway acts as SRST), or a user-defined SRST reference that names a specific gateway.
    System Configuration Guide for Cisco Unified Communications Manager, Release 15 and SUs - Configure SRST · Configure SRST > SRST Overview · Checked 2026-09-25
  11. 11
    In Unified CM, an SRST reference is assigned per device pool, and every phone in that pool tries to connect to the assigned SRST gateway when it cannot reach any Unified CM node.
    System Configuration Guide for Cisco Unified Communications Manager, Release 15 and SUs - Configure SRST · Configure SRST > SRST Overview / Configure SRST Reference; Device Pool > Roaming Sensitive Settings · Checked 2026-09-25
  12. 12
    Engineers report that voice register global runs in SRST mode by default, which is why no 'mode srst' keyword exists and only cme or esrst can be set.field report
    i cant find command SRST mode under voice register global its only mode CME · Thread title and search-result snippet of the accepted reply · Checked 2026-09-25
  13. 13
    Cisco Unified SRST provides fallback call control for Cisco IP phones at a remote site when their connection to primary call control (Unified CM) is lost, whether from a WAN outage or a Unified CM failure.
    Cisco Unified SRST Administration Guide (All Versions) - Cisco Unified SRST Feature Overview · Cisco Unified SRST Feature Overview > Information About Cisco Unified SRST · Checked 2026-09-25
  14. 14
    dialplan-pattern expands abbreviated extensions into fully qualified E.164 numbers during fallback, and up to five patterns can be configured.
    Cisco Unified SRST Administration Guide (All Versions) - Configuring Call Handling · Configuring Call Handling > Global Prefixes (dialplan-pattern) · Checked 2026-09-25
  15. 15
    When a SIP phone registers and matches a voice register pool that has a proxy configured, SRST automatically creates two dial peers: one routing to the proxy and a fallback dial peer routing to the phone.
    Cisco Unified SRST Administration Guide (All Versions) - Setting Up Cisco Unified IP Phones using SIP · Setting Up Cisco Unified IP Phones using SIP > voice register pool (proxy) · Checked 2026-09-25
  16. 16
    E-SRST adds features beyond classic SRST, including advanced voice hunt groups (HLog, statistics, mixed SIP/SCCP members), shared lines, BLF monitoring, video calls and basic ACD (B-ACD).
    Cisco Unified SRST Administration Guide (All Versions) - Enhanced SRST · Enhanced SRST > Information About Enhanced SRST / feature list · Checked 2026-09-25
  17. 17
    Unified E-SRST requires one SRST_E_EP license entitlement for each configured SIP or SCCP phone.
    Cisco Unified SRST Administration Guide (All Versions) - Enhanced SRST · Enhanced SRST > Licensing · Checked 2026-09-25
  18. 18
    Enhanced SRST is enabled with mode esrst under telephony-service for SCCP phones and under voice register global for SIP phones.
    Cisco Unified SRST Administration Guide (All Versions) - Enhanced SRST · Enhanced SRST > Configure Enhanced SRST mode · Checked 2026-09-25
  19. 19
    When the connection to Unified CM is restored, the IP phones automatically cancel their registration with the SRST router and re-register with Unified CM.
    Cisco Unified SRST Administration Guide (All Versions) - Cisco Unified SRST Feature Overview · Cisco Unified SRST Feature Overview > Cisco Unified IP Phone Fallback (restoration) · Checked 2026-09-25
  20. 20
    With default settings, a fallback-return test should expect idle phones to stay on the SRST router for about two minutes after the WAN is restored, and phones on active calls to stay until those calls end.inferred
    System Configuration Guide for Cisco Unified Communications Manager, Release 15 and SUs - Configure SRST · Configure SRST > Connection Monitor Duration; combined with SRST Feature Overview restoration text · Checked 2026-09-25
  21. 21
    The SRST feature roadmap for release 15.0 says a Collaboration Flex subscription is required for ongoing support, alongside the switch to reporting a flex subscription entitlement tag.
    Cisco Unified SRST Administration Guide (All Versions) - Cisco Unified Survivable Remote Site Telephony Feature Roadmap · Feature Roadmap > Unified SRST 15.0 row · Checked 2026-09-25
  22. 22
    Under call-manager-fallback, ip source-address sets the address and port (default 2000) on which the router accepts SCCP phone messages, and the strict-match keyword enforces strict IP address verification.
    Cisco Unified SRST Administration Guide (All Versions) - Setting Up Cisco Unified IP Phones using SCCP · Setting Up Cisco Unified IP Phones using SCCP > Configure SRST Support for Phones (ip source-address step) · Checked 2026-09-25
  23. 23
    SRST license usage is reported three minutes after the last configuration change.
    Cisco Unified SRST Administration Guide (All Versions) - Cisco Unified SIP SRST on Cisco 4000 Series Integrated Services Router · Cisco Unified SIP SRST on Cisco 4000 Series ISR > Licensing / Smart Licensing · Checked 2026-09-25
  24. 24
    Unified SRST requires one SRST_EP license entitlement for each configured SIP or SCCP phone.
    Cisco Unified SRST Administration Guide (All Versions) - Cisco Unified SIP SRST on Cisco 4000 Series Integrated Services Router · Cisco Unified SIP SRST on Cisco 4000 Series ISR > Licensing / Smart Licensing · Checked 2026-09-25
  25. 25
    Starting with Cisco IOS XE 17.18.1a, Smart Licensing reports SRST usage with the flex subscription entitlement tag SRST_EP_SUB across all supported platforms.
    Cisco Unified SRST Administration Guide (All Versions) - Cisco Unified SIP SRST on Cisco 4000 Series Integrated Services Router · Cisco Unified SIP SRST on Cisco 4000 Series ISR > Licensing / Smart Licensing · Checked 2026-09-25
  26. 26
    For SRST/E-SRST 14.3, the Catalyst 8000V supports 500 phones (small), 1,000 phones (medium) or 2,000 phones (large), with 2,000 lines in each size.
    Unified SRST/E-SRST 14.3 Supported Firmware, Platforms, Memory, and Voice Products · Platform capacity table > rows Catalyst 8000V Small/Medium/Large · Checked 2026-09-25
  27. 27
    For SRST/E-SRST 14.3, the C8200-1N-4T and all four listed C8300 models support 2,500 phones and 3,500 lines, while the C8200L-1N-4T supports 1,500 phones and 2,500 lines.
    Unified SRST/E-SRST 14.3 Supported Firmware, Platforms, Memory, and Voice Products · Platform capacity table > rows C8200L-1N-4T, C8200-1N-4T, C8300-* · Checked 2026-09-25
  28. 28
    For SRST/E-SRST 14.3, the ISR 4321 supports up to 50 phones and 200 phone lines (the same as the ISR 1100).
    Unified SRST/E-SRST 14.3 Supported Firmware, Platforms, Memory, and Voice Products · Platform capacity table > rows Cisco ISR 1100, Cisco ISR 4321 · Checked 2026-09-25
  29. 29
    For SRST/E-SRST 14.3, the ISR 4331 supports 100 phones / 400 lines and the ISR 4351 supports 700 phones / 1,000 lines.
    Unified SRST/E-SRST 14.3 Supported Firmware, Platforms, Memory, and Voice Products · Platform capacity table > rows Cisco ISR 4331, Cisco ISR 4351 · Checked 2026-09-25
  30. 30
    For SRST/E-SRST 14.3, the ISR 4431 supports 1,200 phones and 1,800 lines.
    Unified SRST/E-SRST 14.3 Supported Firmware, Platforms, Memory, and Voice Products · Platform capacity table > row Cisco ISR 4431 · Checked 2026-09-25
  31. 31
    The Cisco Unified SRST data sheet lists the Cisco 4451-X ISR at 2,000 phones and 3,500 lines.disputed
    Cisco Unified Survivable Remote Site Telephony Data Sheet · Data sheet > platform capacity table, row Cisco 4451-X ISR · Checked 2026-09-25
  32. 32
    The 14.3 compatibility document lists the ISR 4451 and ISR 4461 at 2,000 phones and 2,500 lines, while the 2022 SRST data sheet lists the ISR 4451-X at 2,000 phones and 3,500 lines.disputed
    Unified SRST/E-SRST 14.3 Supported Firmware, Platforms, Memory, and Voice Products · Platform capacity table > rows Cisco ISR 4451, Cisco ISR 4461 · Checked 2026-09-25
  33. 33
    In call-manager-fallback mode, max-ephones and max-dn both default to 0, and their maximum values are platform-dependent.
    Cisco Unified SRST Administration Guide (All Versions) - Setting Up Cisco Unified IP Phones using SCCP · Setting Up Cisco Unified IP Phones using SCCP > max-ephones / max-dn steps · Checked 2026-09-25
  34. 34
    SCCP and SIP phones can be mixed on one SRST router, provided that the combined total does not exceed the platform's listed maximum.
    Unified SRST/E-SRST 14.3 Supported Firmware, Platforms, Memory, and Voice Products · Platform capacity table > footnote on SCCP/SIP mix · Checked 2026-09-25
  35. 35
    Setting mode cme under voice register global makes the router provision SIP phones as a Unified CME system instead of acting as SRST, so it should not be set on a router meant for plain SRST.inferred
    Cisco Unified SRST and Cisco Unified SIP SRST Command Reference (All Versions) - Command Reference: A through M · Command Reference A through M > mode (voice register global) usage guidelines · Checked 2026-09-25
  36. 36
    The mode command under voice register global takes the keywords cme or esrst; no srst keyword is documented.
    Cisco Unified SRST and Cisco Unified SIP SRST Command Reference (All Versions) - Command Reference: A through M · Command Reference A through M > mode (voice register global) · Checked 2026-09-25
  37. 37
    PSTN access during SRST fallback depends on POTS dial peers whose destination-pattern maps dialled digits to local voice ports (FXO, E&M, BRI or PRI).
    Cisco Unified SRST Administration Guide (All Versions) - Configuring Call Handling · Configuring Call Handling > PSTN / dial peers · Checked 2026-09-25
  38. 38
    After max-ephones or max-dn has been set, the router must be rebooted to reduce the limit.
    Cisco Unified SRST Administration Guide (All Versions) - Setting Up Cisco Unified IP Phones using SCCP · Setting Up Cisco Unified IP Phones using SCCP > max-ephones / max-dn notes · Checked 2026-09-25
  39. 39
    Unified SRST/E-SRST 14.3 is supported only on platforms running Cisco IOS XE Cupertino 17.9.1a and later.
    Unified SRST/E-SRST 14.3 Supported Firmware, Platforms, Memory, and Voice Products · Unified SRST/E-SRST 14.3 Supported Firmware, Platforms, Memory > Platforms · Checked 2026-09-25
  40. 40
    Unified SRST 14.3 introduced Webex Survivability Gateway mode, which provides survivability for Webex Calling endpoints as well as Unified CM-registered endpoints.
    Cisco Unified SRST Administration Guide (All Versions) - Cisco Unified Survivable Remote Site Telephony Feature Roadmap · Feature Roadmap > Unified SRST 14.3 row · Checked 2026-09-25
  41. 41
    Unified SRST 14.4 is included in Cisco IOS XE 17.14.1a and later and added TLS 1.3 support for secure SIP and SCCP SRST.
    Release Notes for Cisco Unified Survivable Remote Site Telephony, 14.4 · Release Notes 14.4 > Introduction / New and Changed Features · Checked 2026-09-25
  42. 42
    The SRST feature roadmap lists Unified SRST 15.0 against Cisco IOS XE 17.18.1a, 17.18.2 and a release labelled 26.1.1, adding Cisco Desk Phone 9800 Series support and TLS compliance controls.
    Cisco Unified SRST Administration Guide (All Versions) - Cisco Unified Survivable Remote Site Telephony Feature Roadmap · Feature Roadmap > Unified SRST 15.0 row · Checked 2026-09-25
  43. 43
    SCCP SRST is verified with show call-manager-fallback all (feature enabled) and show ephone (phones registered to the SRST router).
    Cisco Unified SRST Administration Guide (All Versions) - Setting Up Cisco Unified IP Phones using SCCP · Setting Up Cisco Unified IP Phones using SCCP > Verify · Checked 2026-09-25
  44. 44
    The Unified SRST restrictions state that all calls must use the same codec, either G.711 or G.729r8.
    Cisco Unified SRST Administration Guide (All Versions) - Cisco Unified SRST Feature Overview · Cisco Unified SRST Feature Overview > Restrictions · Checked 2026-09-25
  45. 45
    Classic Unified SIP SRST does not support BLF speed-dial notification or call-forward synchronization, and music on hold is not supported for calls that SIP phones put on hold.
    Cisco Unified SRST Administration Guide (All Versions) - Cisco Unified SRST Feature Overview · Cisco Unified SRST Feature Overview > Restrictions for Cisco Unified SIP SRST · Checked 2026-09-25
  46. 46
    Unified SIP SRST on the ISR 4000 was introduced in Unified SRST 10.0 (IOS XE 3.10S) and extended to all ISR 4000 models listed in the chapter in Unified SRST 10.5 (IOS XE 3.13S).
    Cisco Unified SRST Administration Guide (All Versions) - Cisco Unified SIP SRST on Cisco 4000 Series Integrated Services Router · Cisco Unified SIP SRST on Cisco 4000 Series ISR > Feature information / supported platforms · Checked 2026-09-25
  47. 47
    SIP SRST is enabled under voice service voip with allow-connections sip to sip and, in the sip sub-mode, the registrar server command with optional expires max and min values.
    Cisco Unified SRST Administration Guide (All Versions) - Cisco Unified SIP SRST on Cisco 4000 Series Integrated Services Router · Cisco Unified SIP SRST on Cisco 4000 Series ISR > configuration task (voice service voip) · Checked 2026-09-25
  48. 48
    Unified SIP SRST on the ISR 4000 does not support transcoding, shared lines or mixed shared lines, video calling, or multicast MOH for SIP.
    Cisco Unified SRST Administration Guide (All Versions) - Cisco Unified SIP SRST on Cisco 4000 Series Integrated Services Router · Cisco Unified SIP SRST on Cisco 4000 Series ISR > Restrictions · Checked 2026-09-25
  49. 49
    For SIP phones, Unified SIP SRST backs up an external SIP call control (IP-PBX) by providing basic registrar and redirect server or back-to-back user agent (B2BUA) services.
    Cisco Unified SRST Administration Guide (All Versions) - Cisco Unified SRST Feature Overview · Cisco Unified SRST Feature Overview > Cisco Unified SIP SRST · Checked 2026-09-25
  50. 50
    The documented SIP SRST verification commands include show sip-ua status registrar, show voice register dial-peers, show dial-peer voice and show license summary.
    Cisco Unified SRST Administration Guide (All Versions) - Cisco Unified SIP SRST on Cisco 4000 Series Integrated Services Router · Cisco Unified SIP SRST on Cisco 4000 Series ISR > Verifying · Checked 2026-09-25
  51. 51
    Smart Licensing Using Policy was introduced for Unified SRST in release 14.1 (IOS XE 17.3.2 through 17.6.1a timeframe).
    Cisco Unified SRST Administration Guide (All Versions) - Cisco Unified Survivable Remote Site Telephony Feature Roadmap · Feature Roadmap > Unified SRST 14.1 row · Checked 2026-09-25
  52. 52
    From Unified SRST 12.5 (IOS XE 16.10.1a), paper Right-To-Use licenses are no longer supported and Smart Licensing entitlements are mandatory.
    Cisco Unified Survivable Remote Site Telephony Data Sheet · Data sheet > Licensing · Checked 2026-09-25
  53. 53
    Cisco SRST can be bought as part of a Cisco Collaboration Flex subscription, where the entitlements come with on-premises calling licenses.
    Cisco Unified Survivable Remote Site Telephony Data Sheet · Data sheet > Licensing · Checked 2026-09-25
  54. 54
    Cisco Unified SRST Manager is end-of-life, and E-SRST is now provisioned only manually through the CLI.
    Cisco Unified SRST Administration Guide (All Versions) - Enhanced SRST · Enhanced SRST > provisioning note · Checked 2026-09-25
  55. 55
    The TLS 1.3 support added in SRST 14.4 applies to SCCP analog voice gateways but not to SCCP IP phone endpoints.
    Cisco Unified SRST Administration Guide (All Versions) - Cisco Unified Survivable Remote Site Telephony Feature Roadmap · Feature Roadmap > Unified SRST 14.4 row (note) · Checked 2026-09-25
  56. 56
    SIP SRST capacity is set under voice register global with max-pool (maximum SIP phone pools) and max-dn, and max-pool is platform-dependent.
    Cisco Unified SRST Administration Guide (All Versions) - Cisco Unified SIP SRST on Cisco 4000 Series Integrated Services Router · Cisco Unified SIP SRST on Cisco 4000 Series ISR > voice register global configuration · Checked 2026-09-25
  57. 57
    A voice register pool uses id network/mask, id ip/mask or id mac to decide which SIP phone registrations the SRST router accepts, and can carry dial-peer attributes such as dtmf-relay and voice-class codec.
    Cisco Unified SRST Administration Guide (All Versions) - Setting Up Cisco Unified IP Phones using SIP · Setting Up Cisco Unified IP Phones using SIP > Configure Backup Registrar / voice register pool · Checked 2026-09-25
  58. 58
    Webex Calling registrations and call control revert from the Survivability Gateway to the Webex cloud once the Webex connection has been restored for at least 30 seconds.
    Site survivability for Webex Calling · Site survivability for Webex Calling > How site survivability works · Checked 2026-09-25
  59. 59
    The Webex Calling Survivability Gateway requires Cisco IOS XE Dublin 17.12.3 or later and must use an IPv4 address, because IPv6 is not supported.
    Site survivability for Webex Calling · Site survivability for Webex Calling > Requirements / Restrictions · Checked 2026-09-25
  60. 60
    For Webex Calling, the Survivability Gateway is enabled with mode webex-sgw under voice register global, and the Webex cloud includes the gateway address, hostname and port in the device configuration instead of relying on a Unified CM SRST reference.
    Site survivability for Webex Calling · Site survivability for Webex Calling > configuration (voice register global) and How it works · Checked 2026-09-25

Documents

tier 2 current vendor documentation

Cisco Unified SRST Administration Guide (All Versions) - Cisco Unified SIP SRST on Cisco 4000 Series Integrated Services Router

Cisco Systems · 2026-04-25 · accessed 2026-09-25

tier 2 current vendor documentation

Cisco Unified SRST Administration Guide (All Versions) - Cisco Unified SRST Feature Overview

Cisco Systems · 2026-04-25 · accessed 2026-09-24

tier 2 current vendor documentation

Cisco Unified SRST Administration Guide (All Versions) - Cisco Unified Survivable Remote Site Telephony Feature Roadmap

Cisco · 2026-04-25 · accessed 2026-09-25

tier 2 current vendor documentation

Cisco Unified SRST Administration Guide (All Versions) - Configuring Call Handling

Cisco Systems · 2026-04-25 · accessed 2026-09-25

tier 2 current vendor documentation

Cisco Unified SRST Administration Guide (All Versions) - Enhanced SRST

Cisco Systems · 2026-04-25 · accessed 2026-09-24

tier 2 current vendor documentation

Cisco Unified SRST Administration Guide (All Versions) - Setting Up Cisco Unified IP Phones using SCCP

Cisco Systems · 2026-04-25 · accessed 2026-09-25

tier 2 current vendor documentation

Cisco Unified SRST Administration Guide (All Versions) - Setting Up Cisco Unified IP Phones using SIP

Cisco Systems · 2026-04-25 · accessed 2026-09-25

tier 2 current vendor documentation

Cisco Unified SRST and Cisco Unified SIP SRST Command Reference (All Versions) - Command Reference: A through M

Cisco Systems · 2026-04-21 · accessed 2026-09-25

tier 2 current vendor documentation

Cisco Unified Survivable Remote Site Telephony Data Sheet

Cisco Systems · 2022-10-21 · accessed 2026-09-25

tier 2 current vendor documentation

Release Notes for Cisco Unified Survivable Remote Site Telephony, 14.4

Cisco Systems · 2024-03-30 · accessed 2026-09-25

tier 2 current vendor documentation

Site survivability for Webex Calling

Cisco Systems, Inc. (help.webex.com) · 2026-07-06 · accessed 2026-09-16

tier 2 current vendor documentation

System Configuration Guide for Cisco Unified Communications Manager, Release 15 and SUs - Configure SRST

Cisco Systems · 2026-09-22 · accessed 2026-09-24

tier 2 current vendor documentation

Unified SRST/E-SRST 14.3 Supported Firmware, Platforms, Memory, and Voice Products

Cisco Systems · 2023-11-30 · accessed 2026-09-25

tier 6 community and field reports

i cant find command SRST mode under voice register global its only mode CME

Cisco Community · accessed 2026-09-25

Cite this page

APA

WarmTransfer. (2026, September 25). SRST on Cisco IOS XE. WarmTransfer. https://warmtransfer.net/knowledge/srst-configuration

BibTeX

@misc{warmtransfer-srst-configuration,
  title  = {SRST on Cisco IOS XE},
  author = {{WarmTransfer}},
  year   = {2026},
  url    = {https://warmtransfer.net/knowledge/srst-configuration},
  note   = {Verified 2026-09-25}
}