SIP response codes
Verified 2026-09-15 · 21 claims · sources tier 1–2
Also known as 488 Not Acceptable Here, 503 Service Unavailable.
SIP response codes fall into 6 classes: 1xx provisional, 2xx success, 3xx redirection, 4xx request failure, 5xx server failure, and 6xx global failure 19. A 1xx response is provisional, whereas every other class is final, and a final response completes the transaction 18.
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 7. 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 8.
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 10. 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 13. 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 15.
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 16. 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 17.
See also
See also SIP transactions dialogs and call setup. See also SDP offer answer and codec negotiation. See also SIP retransmission timers and transaction state.
Applicability
Across the claims in this article, the evidence covers IETF Session Initiation Protocol, verified as of 2026-09-15. The deployment model is not covered by any claim in this article.
What remains uncertain
Which response codes a Webex trace actually carries is not covered by any claim in this article. Which response code a vendor sends for a condition the specification does not name is not covered by any claim in this article. How often a 488 arrives without the media description that explains it is not covered by any claim in this article. What the reason phrase beside a 480 carries in practice is not covered by any claim in this article.
See also
Explains
- Trunks route groups and dial plans in Control Hub — Failover behaviour is defined in terms of SIP responses
Related to
- ITU-T Q.850 call clearing cause valuesstub — The cause vocabulary a SIP status code is mapped onto in a gateway or a call detail record
- SIP transactions and dialogs — A final response of any class completes the transaction
Referenced by
- ITU-T Q.850 call clearing cause valuesstub — Cause 13 is defined as SIP behaviour and CUCM SIP cause codes carry a Q.850 value in their low word
- SIP retransmission timers and transaction statestub — A provisional response stops retransmission
Claims
| # | Claim | Status | Confidence | Verified |
|---|---|---|---|---|
| 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. Call Detail Records Administration Guide for Cisco Unified Communications Manager Release 12.5(1), complete book, Internet Archive capture of 2025-03-23 · Call Detail Records chapter, Table 13, the two service unavailable rows | fact | 0.90 | 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. Call Detail Records Administration Guide for Cisco Unified Communications Manager Release 12.5(1), complete book, Internet Archive capture of 2025-03-23 · Call Detail Records chapter, Table 13 Cisco-Specific Call Termination Cause Codes | field-pattern | 0.90 | 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. Call Detail Records Administration Guide for Cisco Unified Communications Manager Release 12.5(1), complete book, Internet Archive capture of 2025-03-23 · Call Detail Records chapter, Table 13 Cisco-Specific Call Termination Cause Codes | field-pattern | 0.90 | 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. 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 · Table 1 row for cause 13 and clause 6.2.7.1.10 Cause No. 13 | fact | 0.90 | 2026-09-15 |
| 5 | 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. SIP: Session Initiation Protocol (RFC 3261) · section-21.4.1 | fact | 0.95 | 2026-09-04 |
| 6 | 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. SIP: Session Initiation Protocol (RFC 3261) · section-21.4.2 | fact | 0.95 | 2026-09-04 |
| 7 | A 403 Forbidden response means the server understood the request but refuses to fulfill it, and authorization will not help since the request should not be repeated as-is. SIP: Session Initiation Protocol (RFC 3261) · section-21.4.4 | fact | 0.95 | 2026-09-04 |
| 8 | A 404 Not Found response means the server has definitive information that the user does not exist at the domain in the Request-URI, or that the domain is not handled by the recipient. SIP: Session Initiation Protocol (RFC 3261) · section-21.4.5 | fact | 0.95 | 2026-09-04 |
| 9 | 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. SIP: Session Initiation Protocol (RFC 3261) · section-21.4.9 | fact | 0.95 | 2026-09-04 |
| 10 | A 480 Temporarily Unavailable response means the callee's system was reached but the callee is currently unavailable, such as not logged in or in a do-not-disturb state; the response may include a Retry-After header suggesting a better time to call. SIP: Session Initiation Protocol (RFC 3261) · section-21.4.18 | fact | 0.95 | 2026-09-04 |
| 11 | 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. SIP: Session Initiation Protocol (RFC 3261) · section-21.4.19 | fact | 0.95 | 2026-09-04 |
| 12 | 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. SIP: Session Initiation Protocol (RFC 3261) · section-21.4.21 | fact | 0.95 | 2026-09-04 |
| 13 | A 486 Busy Here response means the callee's system was reached but the callee is not willing or able to accept additional calls right now, though 600 Busy Everywhere should be used instead when no other endpoint could answer. SIP: Session Initiation Protocol (RFC 3261) · section-21.4.24 | fact | 0.95 | 2026-09-04 |
| 14 | 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. SIP: Session Initiation Protocol (RFC 3261) · section-21.4.25 | fact | 0.95 | 2026-09-04 |
| 15 | A 488 Not Acceptable Here response indicates that the request was received but some aspect of the session description or Request-URI was not acceptable to the specific resource addressed; the response may carry a Warning header describing why. SIP: Session Initiation Protocol (RFC 3261) · section-21.4.26 | fact | 0.95 | 2026-09-04 |
| 16 | A 503 Service Unavailable response means the server is temporarily unable to process the request due to overload or maintenance; it may include a Retry-After header, and without one the client should treat it like a 500 Server Internal Error. SIP: Session Initiation Protocol (RFC 3261) · section-21.5.4 | fact | 0.95 | 2026-09-04 |
| 17 | A 603 Decline response means the callee's machine was reached but the user explicitly does not wish to or cannot participate, and it is used only when the client knows that no other endpoint will answer the request. SIP: Session Initiation Protocol (RFC 3261) · section-21.6.2 | fact | 0.95 | 2026-09-04 |
| 18 | A 1xx response is provisional and every other class is final and a final response completes the transaction. RFC 3261 — SIP: Session Initiation Protocol · RFC 3261 sections 17 and 21 | fact | 0.90 | 2026-09-15 |
| 19 | SIP response codes fall into 6 classes: 1xx provisional 2xx success 3xx redirection 4xx request failure 5xx server failure and 6xx global failure. RFC 3261 — SIP: Session Initiation Protocol · RFC 3261 section 21 | fact | 0.90 | 2026-09-15 |
| 20 | 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. Cisco Preferred Architecture for Webex Calling — Design Overview · Registration-based Local Gateway, page 23 | fact | 0.90 | 2026-09-04 |
| 21 | 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. Cisco Preferred Architecture for Webex Calling — Design Overview · Trunks and Route Groups, page 21 | fact | 0.90 | 2026-09-04 |
Sources
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
RFC 3261 — SIP: Session Initiation Protocol
SIP: Session Initiation Protocol (RFC 3261)
Call Detail Records Administration Guide for Cisco Unified Communications Manager Release 12.5(1), complete book, Internet Archive capture of 2025-03-23
Cisco Preferred Architecture for Webex Calling — Design Overview
Cite this page
APA
WarmTransfer. (2026, September 15). SIP response codes. WarmTransfer. https://warmtransfer.net/knowledge/sip-response-codes
BibTeX
@misc{warmtransfer-sip-response-codes,
title = {SIP response codes},
author = {{WarmTransfer}},
year = {2026},
url = {https://warmtransfer.net/knowledge/sip-response-codes},
note = {Verified 2026-09-15}
}