911 callbacks that fail to reach the caller

See also

Sources

1. 1
   In Cisco's documented Emergency Responder callback flow, a CUCM translation pattern prefixes 913 to the inbound ELIN, a 913 CTI route point hands the call to CER, and CER strips the prefix before looking up the ELIN.

   Understand Emergency Responder · PSAP callback flow steps 3 to 5 · Checked 2026-10-14
2. 2
   Cisco states that a Cisco Emergency Responder ELIN can be any number but must be a DID that routes to the CUCM environment so the PSAP callback can return.

   Understand Emergency Responder · ELIN definition and PSAP callback flow step 2 · Checked 2026-10-14
3. 3
   Cisco Emergency Responder matches the callback ELIN against its call history and transfers the call back to CUCM using the directory number of the phone that placed the 911 call.

   Understand Emergency Responder · PSAP callback flow steps 6 to 8 · Checked 2026-10-14
4. 4
   47 CFR 9.11(b)(2) requires interconnected VoIP providers to transmit ANI to the designated answering point and to route all 911 calls using ANI and, if necessary, pseudo-ANI.

   47 CFR § 9.11 - E911 Service (interconnected VoIP) · § 9.11(b)(2)(ii)(B) and (b)(2)(iii) · Checked 2026-10-14
5. 5
   47 CFR 9.16(b)(2) requires MLTS notification to a central location or other person where the system can be configured for it without hardware or software improvements, begun at the same time as the 911 call when technically feasible, without delaying the call.

   47 CFR § 9.16 - General obligations for MLTS (Cornell LII republication) · § 9.16(b)(2)(i) to (iii) · Checked 2026-10-14
6. 6
   47 CFR 9.3 defines ANI for 911 purposes as the number that identifies the calling party and may be used as the callback number.

   47 CFR § 9.3 - Definitions (Cornell LII republication) · § 9.3, definition of Automatic Number Identification (ANI) · Checked 2026-10-14
7. 7
   Under 47 CFR 9.3, an MLTS notification must include a valid callback number, except that the callback number may be omitted if it is technically infeasible to provide.

   47 CFR § 9.3 - Definitions (Cornell LII republication) · § 9.3, definition of MLTS notification · Checked 2026-10-14
8. 8
   47 CFR 9.3 defines pseudo ANI as a number with the same number of digits as ANI that is not a North American Numbering Plan directory number and may be used in place of ANI to convey special meaning.

   47 CFR § 9.3 - Definitions (Cornell LII republication) · § 9.3, definition of Pseudo Automatic Number Identification (Pseudo-ANI) · Checked 2026-10-14
9. 9
   Cisco documents that if the 911 caller's phone has call forward enabled, a Cisco Unified CME PSAP callback reaches the forwarded-to party instead of the caller.

   Cisco Unified Communications Manager Express System Administrator Guide - Enhanced 911 Services · Enhanced 911 Services chapter, restrictions · Checked 2026-10-14
10. 10
    Cisco documents that a Cisco Unified CME PSAP callback does not reach the caller if a blocking feature such as Do Not Disturb is active on the phone.

    Cisco Unified Communications Manager Express System Administrator Guide - Enhanced 911 Services · Enhanced 911 Services chapter, restrictions · Checked 2026-10-14
11. 11
    In Cisco Unified CME, if an ELIN is also used as a real phone number, ordinary outside callers to that number can be connected to the last 911 caller.

    Cisco Unified Communications Manager Express System Administrator Guide - Enhanced 911 Services · Enhanced 911 Services chapter, restrictions · Checked 2026-10-14
12. 12
    In Cisco Unified CME, a PSAP callback to an ELIN whose caller history has expired or was lost to a reboot is sent to the configured callback number, such as a company operator or help desk.

    Cisco Unified Communications Manager Express System Administrator Guide - Enhanced 911 Services · Enhanced 911 Services chapter, callback number under voice emergency response settings and callback processing · Checked 2026-10-14
13. 13
    In Cisco Unified CME Enhanced 911, the Last Caller table entry that maps an ELIN back to the 911 caller expires after 180 minutes by default, configurable from 2 to 2880 minutes with the expiry command.

    Cisco Unified Communications Manager Express System Administrator Guide - Enhanced 911 Services · Enhanced 911 Services chapter, expiry timer and voice emergency response settings configuration · Checked 2026-10-14
14. 14
    In Cisco Unified CME, if no caller history and no fallback destination are available, the PSAP hears reorder tone and the router logs %E911_NO_CALLER: Unable to contact last 911 caller.

    Cisco Unified Communications Manager Express System Administrator Guide - Enhanced 911 Services · Enhanced 911 Services chapter, callback processing and troubleshooting · Checked 2026-10-14
15. 15
    Cisco Unified CME keeps only the most recent 911 caller per ERL, so a second 911 call from the same ERL overwrites the first and the PSAP can no longer call back the first caller.

    Cisco Unified Communications Manager Express System Administrator Guide - Enhanced 911 Services · Enhanced 911 Services chapter, Last Caller table description · Checked 2026-10-14
16. 16
    The Cisco Unified CME Last Caller table is not preserved across a router reboot.

    Cisco Unified Communications Manager Express System Administrator Guide - Enhanced 911 Services · Enhanced 911 Services chapter, restrictions · Checked 2026-10-14
17. 17
    In Cisco Unified CME, a PSAP callback to a shared directory number rings every phone sharing it, so someone other than the original caller may answer.

    Cisco Unified Communications Manager Express System Administrator Guide - Enhanced 911 Services · Enhanced 911 Services chapter, restrictions · Checked 2026-10-14
18. 18
    Cisco's CME guide states that 911 calls placed while remote phones are in SRST fallback never reach Cisco Emergency Responder's call history, so later PSAP callbacks route to the default destination configured on CER for that ELIN rather than the original caller.

    Cisco Unified Communications Manager Express System Administrator Guide - Enhanced 911 Services · Enhanced 911 Services chapter, restrictions (SRST and Cisco Emergency Responder) · Checked 2026-10-14
19. 19
    In Cisco Unified CME, if other translation features change the ELIN before callback processing, the system cannot find the caller history for the callback.

    Cisco Unified Communications Manager Express System Administrator Guide - Enhanced 911 Services · Enhanced 911 Services chapter, restrictions · Checked 2026-10-14
20. 20
    In FCC 19-76 the Commission adopted a technical-infeasibility exception for the callback number in MLTS notification and clarified that the callback number need not be a DID to the caller's extension when none is available.

    FCC 19-76 — Implementing Kari's Law and Section 506 of RAY BAUM'S Act, Report and Order, PS Docket No. 18-261 · Report and Order ¶¶ 20 and 24 · Checked 2026-10-14
21. 21
    FCC 19-76 describes E911 as providing PSAPs with the caller's location and a call-back number as part of each 911 call.

    FCC 19-76 — Implementing Kari's Law and Section 506 of RAY BAUM'S Act, Report and Order, PS Docket No. 18-261 · Report and Order ¶ 3 · Checked 2026-10-14
22. 22
    FCC 19-76 gives the example that a hotel without a DID line for each room can provide the number of a central location such as the front desk as the notification callback number.

    FCC 19-76 — Implementing Kari's Law and Section 506 of RAY BAUM'S Act, Report and Order, PS Docket No. 18-261 · Report and Order ¶ 26 · Checked 2026-10-14
23. 23
    FCC 19-76 accepts that the callback number in an MLTS notification can be an internal extension reachable only from inside the enterprise.

    FCC 19-76 — Implementing Kari's Law and Section 506 of RAY BAUM'S Act, Report and Order, PS Docket No. 18-261 · Report and Order ¶ 26 · Checked 2026-10-14
24. 24
    FCC 19-76 cautions that a notification recipient dialing the callback number may interfere while the PSAP is trying to reconnect after a dropped call, and points to dialing by the same method the caller used to reach 911 as the better practice.

    FCC 19-76 — Implementing Kari's Law and Section 506 of RAY BAUM'S Act, Report and Order, PS Docket No. 18-261 · Report and Order ¶ 28 · Checked 2026-10-14
25. 25
    FCC 19-76 notes that the MLTS station placing a 911 call often has no telephone number of its own, so PSAPs often cannot locate the originating station.

    FCC 19-76 — Implementing Kari's Law and Section 506 of RAY BAUM'S Act, Report and Order, PS Docket No. 18-261 · Report and Order ¶ 5 · Checked 2026-10-14
26. 26
    A callback number that resolves to a main number answered by an auto attendant or IVR is a recognised failure mode: Cisco forbids AA or IVR as a Webex ECBN target and Zoom requires a staffed safety destination, which together indicate that an unattended main number does not reliably connect the PSAP to anyone.inferred

    Emergency Callback Number (ECBN) in Control Hub · ECBN options section, read with zoom-library-emergency-call-handling Receptionist or Call Queue section · Checked 2026-10-14
27. 27
    The NENA Knowledge Base defines the Callback Number as a dialable E.164 number the PSAP uses to reach the caller after the emergency call is released, and says it can differ from the Calling Party Number, which sometimes cannot be used to call the caller back.

    Telephone Number - NENA Knowledge Base · Entries Callback Number (CBN) and Calling Party Number (CPN) · Checked 2026-10-14
28. 28
    Pool or ELIN exhaustion is handled differently by platform: Zoom Phone reuses a pool number so the newest caller takes the callback, Cisco Unified CME overwrites the earlier caller per ERL, and Webex Calling falls back to the ECBN; in all three a concurrent second call can stop the first caller's callback reaching them.inferred

    Emergency Call Handling | Zoom Technical Library · Emergency Number Pool section, read with cisco-cme-sysadmin-e911-services Last Caller table and cisco-help-5oxx10-configure-elin ELIN fallback mechanism · Checked 2026-10-14
29. 29
    Because pseudo ANI is by definition not a NANP directory number, a 911 call routed on pseudo ANI does not by itself give the PSAP a dialable number to call the caller back.inferred

    47 CFR § 9.3 - Definitions (Cornell LII republication) · § 9.3, definition of Pseudo-ANI, read with 47 CFR 9.11(b)(2)(iii) · Checked 2026-10-14
30. 30
    Because RFC 7090 marking is SIP-only and PSTN interworking is out of scope, a PSAP callback arriving at an enterprise over a PSTN or SIP trunk cannot be assumed to carry a psap-callback marking, so enterprise systems rely on ELIN or callback-window state to recognise it.inferred

    RFC 7090: Public Safety Answering Point (PSAP) Callback · Section 3.5 and Section 4.2, read with cisco-cme-sysadmin-e911-services callback dial peer section · Checked 2026-10-14
31. 31
    RFC 6881 requirements ED-68 and SP-40 say call features that would block callbacks, such as do-not-disturb and all kinds of call forwarding, must be disabled and should stay disabled for about 30 minutes after the emergency call ends.

    RFC 6881 (BCP 181): Best Current Practice for Communications Services in Support of Emergency Calling · Section 13, requirements ED-68 and SP-40 · Checked 2026-10-14
32. 32
    RFC 6881 requirement ED-69 has an endpoint recognise a callback as a call from the PSAP's domain within a timer after call termination, suggests 5 minutes for that timer, and notes the method is not always reliable.

    RFC 6881 (BCP 181): Best Current Practice for Communications Services in Support of Emergency Calling · Section 13, requirement ED-69 · Checked 2026-10-14
33. 33
    RFC 6881 requirement SP-36 says the device must have a persistent address of record so a later callback can be returned, and acknowledges such a callback may not reach the originating device.

    RFC 6881 (BCP 181): Best Current Practice for Communications Services in Support of Emergency Calling · Section 10, requirement SP-36 · Checked 2026-10-14
34. 34
    RFC 6881 requirements ED-62 and ED-63/SP-34 say the emergency caller's device should present a globally routable Contact URI, for example a GRUU, that stays valid for several minutes after the call ends.

    RFC 6881 (BCP 181): Best Current Practice for Communications Services in Support of Emergency Calling · Section 9.2 requirement ED-62 item 5; Section 10 requirements ED-63 and SP-34 · Checked 2026-10-14
35. 35
    RFC 6881 requirement SP-35 says callbacks to the device's Contact URI received within 30 minutes of an emergency call must reach the device even if features such as do-not-disturb or call forwarding would normally redirect them.

    RFC 6881 (BCP 181): Best Current Practice for Communications Services in Support of Emergency Calling · Section 10, requirement SP-35 · Checked 2026-10-14
36. 36
    RFC 7090 names call barring, do not disturb and call diversion as features that can prevent a PSAP callback from reaching the emergency caller.

    RFC 7090: Public Safety Answering Point (PSAP) Callback · Section 1 (Introduction) · Checked 2026-10-14
37. 37
    RFC 7090 states that without a marking, SIP entities cannot tell a PSAP callback apart from other calls, so it may be blocked by user-configured authorization policies or forwarded to an answering machine.

    RFC 7090: Public Safety Answering Point (PSAP) Callback · Section 1 (Introduction) · Checked 2026-10-14
38. 38
    RFC 7090 states that the RFC 6443 approach of recognising a callback by matching it against the PSAP domain of the earlier emergency call does not work in all deployments, citing routing asymmetry, multi-stage routing, call forwarding and network-based URN resolution.

    RFC 7090: Public Safety Answering Point (PSAP) Callback · Section 1 and Sections 3.1 to 3.4 · Checked 2026-10-14
39. 39
    RFC 7090 leaves the actual treatment of a marked PSAP callback, such as bypassing call features or barring, to local policy and outside the scope of the specification.

    RFC 7090: Public Safety Answering Point (PSAP) Callback · Section 4.2 · Checked 2026-10-14
40. 40
    RFC 7090 warns that the psap-callback marking can be inserted by any SIP entity, including attackers, to bypass blacklists and call forwarding.

    RFC 7090: Public Safety Answering Point (PSAP) Callback · Sections 5.1 and 5.2 · Checked 2026-10-14
41. 41
    RFC 7090 defines the value psap-callback for the SIP Priority header field to mark a request as a PSAP callback.

    RFC 7090: Public Safety Answering Point (PSAP) Callback · Section 4.1 and Section 4.3.2 (ABNF) · Checked 2026-10-14
42. 42
    RFC 7090 explicitly places PSTN interworking for PSAP callbacks out of scope.

    RFC 7090: Public Safety Answering Point (PSAP) Callback · Section 3.5 · Checked 2026-10-14
43. 43
    RFC 7090 requires VoIP providers to give PSAP callbacks preferential treatment only after the PSAP identity matches a white list, and otherwise to remove the marking.

    RFC 7090: Public Safety Answering Point (PSAP) Callback · Section 5.3 · Checked 2026-10-14
44. 44
    For Teams Phone Direct Routing with an SBC ELIN application, the application substitutes the caller's number with the ELIN on the 911 call and performs the reverse called-number substitution on a callback to reach the original caller.

    Considerations for Direct Routing · Emergency call routing for Direct Routing > Emergency Location Identification Number applications · Checked 2026-10-14
45. 45
    When a user or number used as a Webex Calling ECBN is deleted, Control Hub warns that the ECBN is in use and affected users revert to their own number or, if they have none, the location default ECBN.

    Emergency Callback Number (ECBN) in Control Hub · Deletion and warning behaviour section · Checked 2026-10-14
46. 46
    In Webex Calling, extension-only users do not see the User's phone number ECBN option and should be set to the location default ECBN or an assigned number from their location.

    Emergency Callback Number (ECBN) in Control Hub · Users without a phone number guidance · Checked 2026-10-14
47. 47
    Cisco says the Webex Calling location default ECBN and assigned-number ECBN should point to a live answering point and cannot be an Auto Attendant, IVR or other service.

    Emergency Callback Number (ECBN) in Control Hub · ECBN options section, location default and assigned number guidance · Checked 2026-10-14
48. 48
    Webex Calling offers three ECBN choices for a user, workspace or virtual line: its own phone number, the location default ECBN, or an assigned number of another member at the same location.

    Emergency Callback Number (ECBN) in Control Hub · ECBN options section · Checked 2026-10-14
49. 49
    In Webex Calling, a PSAP callback to an expired ELIN goes to the location's ECBN if one is assigned, and is otherwise rejected with User Not Found.

    Configure ELIN for Webex Calling · ELIN fallback mechanism · Checked 2026-10-14
50. 50
    Webex Calling ELIN assignments expire after 60 minutes by default, configurable per location from 10 to 1440 minutes, and a PSAP callback does not extend the assignment.

    Configure ELIN for Webex Calling · ELIN assignment duration; Configure ELIN expiry time · Checked 2026-10-14
51. 51
    In Webex Calling, a PSAP callback to an actively assigned ELIN routes immediately to the assigned user or workspace with call forwarding, voicemail and similar services temporarily disabled.

    Configure ELIN for Webex Calling · How ELIN works, Emergency call flow using ELIN · Checked 2026-10-14
52. 52
    Cisco states that with Local Gateway legacy behavior enabled, inbound Webex Calling ELIN callbacks through the Local Gateway fail, while outbound ELIN use is unaffected.

    Configure ELIN for Webex Calling · ELIN supported options and limitations, operational constraints · Checked 2026-10-14
53. 53
    In Webex Calling, when no ELIN is available in the location pool or ELIN is not permitted, the emergency call uses the configured ECBN instead.

    Configure ELIN for Webex Calling · ELIN fallback mechanism · Checked 2026-10-14
54. 54
    Webex Calling ELIN is not supported for Webex Go mobile devices or network-interface-originated calls such as Single Number Reach.

    Configure ELIN for Webex Calling · ELIN supported options and limitations, unsupported list · Checked 2026-10-14
55. 55
    Cisco notes that emergency calls from Webex soft clients on cellular devices may go to the native dialer and bypass Webex Calling, and recommends that soft-client users have their own phone number for ECBN.

    Emergency Callback Number (ECBN) in Control Hub · Non-fixed and soft client users guidance · Checked 2026-10-14
56. 56
    Control Hub warns when a toll-free main number is used as a Webex Calling ECBN because toll-free numbers have no emergency service address.

    Emergency Callback Number (ECBN) in Control Hub · Location default ECBN guidance · Checked 2026-10-14
57. 57
    When every Zoom Phone emergency pool number is in use, the system reuses one and the most recent caller to use that number becomes the callback destination.

    Emergency Call Handling | Zoom Technical Library · Emergency Number Pool section · Checked 2026-10-14
58. 58
    Zoom Phone's emergency number pool assigns a temporary pool number as outbound caller ID for an extension-only 911 caller, and for about two hours inbound calls to that number route to the originating extension.

    Emergency Call Handling | Zoom Technical Library · Emergency Number Pool section · Checked 2026-10-14
59. 59
    Zoom says that if a dedicated auto receptionist or call queue number is used as the emergency caller ID for extension-only users, that destination must be reserved for safety response and staffed 24x7x365, otherwise callback routing should return to the originating extension.

    Emergency Call Handling | Zoom Technical Library · Receptionist or Call Queue section · Checked 2026-10-14

Documents

Cite this page

WarmTransfer. (2026, October 14). 911 callbacks that fail to reach the caller. WarmTransfer. https://warmtransfer.net/knowledge/psap-callback-failures

@misc{warmtransfer-psap-callback-failures,
  title  = {911 callbacks that fail to reach the caller},
  author = {{WarmTransfer}},
  year   = {2026},
  url    = {https://warmtransfer.net/knowledge/psap-callback-failures},
  note   = {Verified 2026-10-14}
}