# SIP response codes and failure interpretation

Canonical: https://warmtransfer.net/knowledge/sip-response-codes

Last verified: 2026-09-15

SIP response codes fall into 6 classes: 1xx provisional, 2xx success, 3xx redirection, 4xx request failure, 5xx server failure, and 6xx global failure[^20]. A 1xx response is provisional, whereas every other class is final, and a final response completes the transaction[^19].

## Response code classes and failure semantics

A 403 Forbidden indicates that the server understood the request and is refusing to fulfil it, that authorisation will not help, and that the request should not be repeated[^8]. A 404 Not Found means the server has definitive information that the user does not exist at the domain in the request address or that the domain is not one it handles[^9]. 

A 480 Temporarily Unavailable signifies that the destination system was contacted successfully and the user is currently unavailable, such as being not signed in, in a state that precludes communication, or with do not disturb active[^11]. A 486 Busy Here indicates that the destination system was contacted successfully and the user is not willing or able to take additional calls at that end system[^14]. A 488 Not Acceptable Here means the request may succeed elsewhere, applies only to the resource addressed, and may carry a body describing the media capabilities the responder does support[^16].

A 503 Service Unavailable means the server is temporarily unable to process the request through overloading or maintenance, and a client receiving one should try an alternate server[^17]. A 603 Decline means the destination machine was contacted successfully, the user explicitly does not wish to or cannot participate, and it is returned only when the client knows no other endpoint will answer[^18].

## See also

See also [SIP transactions dialogs and call setup](https://warmtransfer.net/knowledge/sip-transactions-dialogs).
See also [SDP offer answer and codec negotiation](https://warmtransfer.net/knowledge/sdp-offer-answer).
See also [SIP retransmission timers and transaction state](https://warmtransfer.net/knowledge/sip-retransmission-timers).

## Applicability

Applies to: IETF Session Initiation Protocol. Sources checked 2026-09-15.

## What remains uncertain

Which response codes a Webex trace actually carries is not covered by the sources below. Which response code a vendor sends for a condition the specification does not name is not covered by the sources below. How often a 488 arrives without the media description that explains it is not covered by the sources below. What the reason phrase beside a 480 carries in practice is not covered by the sources below.

## Sources

[^1]: The Cisco-specific cause code table lists SIP 503 service unavailable twice with two different codes whose Q.850 low words differ, 41 temporary failure and 63 service or option not available unspecified, so one SIP status maps to two Unified CM cause codes. Source: [Call Detail Records Administration Guide for Cisco Unified Communications Manager Release 12.5(1), complete book, Internet Archive capture of 2025-03-23](https://web.archive.org/web/20250323135843id_/https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/cucm/service/12_5_1/cdrdef/cucm_b_cdr-admin-guide-1251.pdf), Call Detail Records chapter, Table 13, the two service unavailable rows. Checked 2026-09-15.
[^2]: Every one of the 42 SIP rows of the Cisco-specific cause code table has a Q.850 cause value in the low 16 bits of its code, and the guide states the construction for one row by writing the loop-detected code as 0x42000000 plus EXCHANGE_ROUTING_ERROR (field report). Source: [Call Detail Records Administration Guide for Cisco Unified Communications Manager Release 12.5(1), complete book, Internet Archive capture of 2025-03-23](https://web.archive.org/web/20250323135843id_/https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/cucm/service/12_5_1/cdrdef/cucm_b_cdr-admin-guide-1251.pdf), Call Detail Records chapter, Table 13 Cisco-Specific Call Termination Cause Codes. Checked 2026-09-15.
[^3]: The high byte of a SIP-derived Unified CM cause code is not a fixed function of the SIP status code: across the 41 distinct status codes in the table the difference between the status code and the marker takes 6 different values, so decoding the SIP status out of a cause code needs the table rather than arithmetic (field report). Source: [Call Detail Records Administration Guide for Cisco Unified Communications Manager Release 12.5(1), complete book, Internet Archive capture of 2025-03-23](https://web.archive.org/web/20250323135843id_/https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/cucm/service/12_5_1/cdrdef/cucm_b_cdr-admin-guide-1251.pdf), Call Detail Records chapter, Table 13 Cisco-Specific Call Termination Cause Codes. Checked 2026-09-15.
[^4]: Cause 13 call completed elsewhere is the one Q.850 cause whose application column reads SIP rather than DSS 1 or ISUP, and its definition is a rule about what an S-CSCF puts in the Reason header field of a CANCEL after a forked INVITE receives a 2xx. Source: [ITU-T Recommendation Q.850 (10/2018), Usage of cause and location in the Digital Subscriber Signalling System No. 1 and the Signalling System No. 7 ISDN user part](https://www.itu.int/rec/dologin_pub.asp?lang=e&id=T-REC-Q.850-201810-I!!PDF-E&type=items), Table 1 row for cause 13 and clause 6.2.7.1.10 Cause No. 13. Checked 2026-09-15.
[^5]: If the answerer has no media format in common for an offered stream it must reject that stream by setting the port to zero, and if there is no format in common for any stream the entire offered session is rejected. Source: [RFC 3264 — An Offer/Answer Model with the Session Description Protocol (SDP)](https://www.rfc-editor.org/rfc/rfc3264.html), RFC 3264 section 6.1 Unicast Streams. Checked 2026-09-15.
[^6]: A 400 Bad Request response means the server could not parse the request because of malformed syntax; the reason phrase should identify the specific syntax problem, such as a missing header field. Source: [SIP: Session Initiation Protocol (RFC 3261)](https://www.rfc-editor.org/rfc/rfc3261), section-21.4.1. Checked 2026-09-04.
[^7]: A 401 Unauthorized response means the request requires user authentication; it is issued by user agent servers and registrars, distinct from 407 which is used by proxy servers. Source: [SIP: Session Initiation Protocol (RFC 3261)](https://www.rfc-editor.org/rfc/rfc3261), section-21.4.2. Checked 2026-09-04.
[^8]: A 403 Forbidden means the server understood the request and is refusing to fulfil it and that authorisation will not help and the request should not be repeated. Source: [RFC 3261 — SIP: Session Initiation Protocol](https://www.rfc-editor.org/rfc/rfc3261.html), RFC 3261 section 21, 403 Forbidden. Checked 2026-09-15.
[^9]: A 404 Not Found means the server has definitive information that the user does not exist at the domain in the request address or that the domain is not one it handles. Source: [RFC 3261 — SIP: Session Initiation Protocol](https://www.rfc-editor.org/rfc/rfc3261.html), RFC 3261 section 21, 404 Not Found. Checked 2026-09-15.
[^10]: A 408 Request Timeout response means the server could not produce a response within a suitable time, for example because it could not locate the user, and the client may retry the unmodified request later. Source: [SIP: Session Initiation Protocol (RFC 3261)](https://www.rfc-editor.org/rfc/rfc3261), section-21.4.9. Checked 2026-09-04.
[^11]: A 480 Temporarily Unavailable means the destination system was contacted successfully and the user is currently unavailable for example not signed in or in a state that precludes communication or with do not disturb active. Source: [RFC 3261 — SIP: Session Initiation Protocol](https://www.rfc-editor.org/rfc/rfc3261.html), RFC 3261 section 21, 480 Temporarily Unavailable. Checked 2026-09-15.
[^12]: A 481 Call/Transaction Does Not Exist response means the UAS received a request that does not match any dialog or transaction it currently knows about. Source: [SIP: Session Initiation Protocol (RFC 3261)](https://www.rfc-editor.org/rfc/rfc3261), section-21.4.19. Checked 2026-09-04.
[^13]: A 483 Too Many Hops response means the server received a request whose Max-Forwards header field had already reached zero, so the request was not forwarded any further. Source: [SIP: Session Initiation Protocol (RFC 3261)](https://www.rfc-editor.org/rfc/rfc3261), section-21.4.21. Checked 2026-09-04.
[^14]: A 486 Busy Here means the destination system was contacted successfully and the user is not willing or able to take additional calls at that end system. Source: [RFC 3261 — SIP: Session Initiation Protocol](https://www.rfc-editor.org/rfc/rfc3261.html), RFC 3261 section 21, 486 Busy Here. Checked 2026-09-15.
[^15]: A 487 Request Terminated response means the original request was terminated by a BYE or CANCEL, and it is never sent as the response to a CANCEL request itself. Source: [SIP: Session Initiation Protocol (RFC 3261)](https://www.rfc-editor.org/rfc/rfc3261), section-21.4.25. Checked 2026-09-04.
[^16]: A 488 Not Acceptable Here means the request may succeed elsewhere and applies only to the resource addressed and may carry a body describing the media capabilities the responder does support. Source: [RFC 3261 — SIP: Session Initiation Protocol](https://www.rfc-editor.org/rfc/rfc3261.html), RFC 3261 section 21, 488 Not Acceptable Here. Checked 2026-09-15.
[^17]: A 503 Service Unavailable means the server is temporarily unable to process the request through overloading or maintenance and a client receiving one should try an alternate server. Source: [RFC 3261 — SIP: Session Initiation Protocol](https://www.rfc-editor.org/rfc/rfc3261.html), RFC 3261 section 21, 503 Service Unavailable. Checked 2026-09-15.
[^18]: A 603 Decline means the destination machine was contacted successfully and the user explicitly does not wish to or cannot participate and is returned only when the client knows no other endpoint will answer. Source: [RFC 3261 — SIP: Session Initiation Protocol](https://www.rfc-editor.org/rfc/rfc3261.html), RFC 3261 section 21, 603 Decline. Checked 2026-09-15.
[^19]: A 1xx response is provisional and every other class is final and a final response completes the transaction. Source: [RFC 3261 — SIP: Session Initiation Protocol](https://www.rfc-editor.org/rfc/rfc3261.html), RFC 3261 sections 17 and 21. Checked 2026-09-15.
[^20]: SIP response codes fall into 6 classes: 1xx provisional 2xx success 3xx redirection 4xx request failure 5xx server failure and 6xx global failure. Source: [RFC 3261 — SIP: Session Initiation Protocol](https://www.rfc-editor.org/rfc/rfc3261.html), RFC 3261 section 21. Checked 2026-09-15.
[^21]: Calls exceeding the registration-based Local Gateway concurrency ceiling are rejected by the Webex Calling access session border controller with a SIP 403 Forbidden response. 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.
[^22]: Rerouting to another trunk is triggered by SIP responses 401, 470, 480 and 606 when no provisional 18x response preceded them, by all other 4xx responses except 403, 404, 410, 413, 484 and 486, by all 5xx responses, and by a 12 second SIP timeout. 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), Trunks and Route Groups, page 21. Checked 2026-09-04.