| Value | Details |
400 Bad Request | The request could not be understood due to malformed syntax. |
401 Unauthorized | The request requires user authentication. This response is issued by UASs and registrars. |
402 Payment Required | Reserved for future use. |
403 Forbidden | The server understood the request, but is refusing to fulfil it |
404 Not Found | The server has definitive information that the user does not exist at the domain specified in the Request-URI. This status is also returned if the domain in the Request-URI does not match any of the domains handled by the recipient of the request. |
405 Method Not Allowed | The method specified in the Request-Line is understood, but not allowed for the address identified by the Request-URI |
406 Not Acceptable | The resource identified by the request is only capable of generating response entities that have content characteristics but not acceptable according to the Accept header field sent in the request |
407 Proxy Authentication Required | The request requires user authentication. This response is issued by proxys |
408 Request Timeout | Couldn't find the user in time. The server could not produce a response within a suitable amount of time, for example, if it could not determine the location of the user in time. The client MAY repeat the request without modifications at any later time |
409 Conflict | User already registered.[9]:§7.4.10 Deprecated by omission from later RFCs[1] and by non-registration with the IANA. |
410 Gone | The user existed once, but is not available here any more. |
411 Length Required | The server will not accept the request without a valid Content-Length.[9]:§7.4.12 Deprecated by omission from later RFC and by non-registration with the IANA |
412 Conditional Request Failed | The given precondition has not been met |
413 Request Entity Too Large | Request body too large |
414 Request-URI Too Long | The server is refusing to service the request because the Request-URI is longer than the server is willing to interpret |
415 Unsupported Media Type | Request body in a format not supported |
416 Unsupported URI Scheme | Request-URI is unknown to the server. |
417 Unknown Resource-Priority | There was a resource-priority option tag, but no Resource-Priority header |
420 Bad Extension | Bad SIP Protocol Extension used, not understood by the server. |
421 Extension Required | The server needs a specific extension not listed in the Supported header. |
422 Session Interval Too Small | The received request contains a Session-Expires header field with a duration below the minimum timer. |
423 Interval Too Brief | Expiration time of the resource is too short. |
424 Bad Location Information | The request's location content was malformed or otherwise unsatisfactory. |
428 Use Identity Header | The server policy requires an Identity header, and one has not been provided. |
429 Provide Referrer Identity | The server did not receive a valid Referred-By token on the request. |
430 Flow Failed | A specific flow to a user agent has failed, although other flows may succeed. This response is intended for use between proxy devices, and should not be seen by an endpoint (and if it is seen by one, should be treated as a 400 Bad Request response) |
433 Anonymity Disallowed | The request has been rejected because it was anonymous. |
436 Bad Identity-Info | The request has an Identity-Info header, and the URI scheme in that header cannot be dereferenced. |
437 Unsupported Certificate | The server was unable to validate a certificate for the domain that signed the request. |
438 Invalid Identity Header | The server obtained a valid certificate that the request claimed was used to sign the request, but was unable to verify that signature. |
439 First Hop Lacks Outbound Support | The first outbound proxy the user is attempting to register through does not support the "outbound" feature of RFC 5626, although the registrar does. |
470 Consent Needed | The source of the request did not have the permission of the recipient to make such a request. |
480 Temporarily Unavailable | Callee currently unavailable. |
481 Call/Transaction Does Not Exist | Server received a request that does not match any dialog or transaction. |
482 Loop Detected. | Server has detected a loop. |
483 Too Many Hops | Max-Forwards header has reached the value '0'. |
484 Address Incomplete | Request-URI incomplete. |
485 Ambiguous | Request-URI is ambiguous. |
486 Busy Here | Callee is busy. |
487 Request Terminated | Request has terminated by bye or cancel |
488 Not Acceptable Here | Some aspect of the session description or the Request-URI is not acceptable |
489 Bad Event | The server did not understand an event package specified in an Event header field. |
491 Request Pending | Server has some pending request from the same dialog. |
493 Undecipherable | Request contains an encrypted MIME body, which recipient can not decrypt |
494 Security Agreement Required | The server has received a request that requires a negotiated security mechanism, and the response contains a list of suitable security mechanisms for the requester to choose between or a digest authentication challenge |