SIP requests and responses may be generated by any SIP user agent; user agents are divided into clients (UACs), which initiate requests, and servers (UASes), which respond to them.:§8 A single user agent may act as both UAC and UAS for different transactions::p26 for example, a SIP phone is a user agent that will be a UAC when making a call, and a UAS when receiving one. Additionally, some devices will act as both UAC and UAS for a single transaction; these are called Back-to-Back User Agents (B2BUAs).:p20
SIP responses specify a three-digit integer response code, which is one of a number of defined codes that detail the status of the request. These codes are grouped according to their first digit as "provisional", "success", "redirection", "client error", "server error" or "global failure" codes, corresponding to a first digit of 1–6; these are expressed as, for example, "1xx" for provisional responses with a code of 100–199.:§7.2 The SIP response codes are an extension to the HTTP response codes, although not all HTTP response codes are valid in SIP.:§21
SIP responses also specify a "reason phrase", and a default reason phrase is defined with each response code.:§7.2 These reason phrases can be varied, however, such as to provide additional information:§21.4.18 or to provide the text in a different language.:§20.3
This list details all the SIP response codes defined in IETFRFCs and registered with the IANA as of 18 April 2013[update]. It also includes SIP response codes defined in obsolete SIP RFCs (specifically, RFC 2543), which are therefore not registered with the IANA; these are explicitly noted as such.
Destination user agent received INVITE, and is alerting user of call.:§21.1.2
181 Call is Being Forwarded
Servers can optionally send this response to indicate a call is being forwarded.:§21.1.3
Indicates that the destination was temporarily unavailable, so the server has queued the call until the destination is available. A server may send multiple 182 responses to update progress of the queue.:§21.1.4
183 Session in Progress
This response may be used to send extra information for a call which is still being set up.:§21.1.5
199 Early Dialog Terminated
Can be used by User Agent Server to indicate to upstream SIP entities (including the User Agent Client (UAC)) that an early dialog has been terminated.
The server understood the request, but is refusing to fulfil it.:§21.4.4
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.:§21.4.5
405 Method Not Allowed
The method specified in the Request-Line is understood, but not allowed for the address identified by the Request-URI.:§21.4.6
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.:§21.4.7
407 Proxy Authentication Required
The request requires user authentication. This response is issued by proxys.:§21.4.8
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. :§21.4.9
User already registered.:§7.4.10 Deprecated by omission from later RFCs and by non-registration with the IANA.
The user existed once, but is not available here any more.:§21.4.10
411 Length Required
The server will not accept the request without a valid Content-Length.:§7.4.12 Deprecated by omission from later RFCs and by non-registration with the IANA.
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.:§21.4.15
421 Extension Required
The server needs a specific extension not listed in the Supported header.:§21.4.16
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.:§21.4.17
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.:p11
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).:§11.5
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.:p11
437 Unsupported Certificate
The server was unable to validate a certificate for the domain that signed the request.:p11
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.:p12
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.:§11.6
470 Consent Needed
The source of the request did not have the permission of the recipient to make such a request.
Request has terminated by bye or cancel.:§21.4.25
488 Not Acceptable Here
Some aspect of the session description or the Request-URI is not acceptable.:§21.4.26
489 Bad Event
The server did not understand an event package specified in an Event header field.:§7.3.2:§8.3.2
491 Request Pending
Server has some pending request from the same dialog.:§21.4.27
Request contains an encrypted MIME body, which recipient can not decrypt.:§21.4.28
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,:§§2.3.1–2.3.2 or a digest authentication challenge.:§2.4
5xx—Server Failure Responses
500 Server Internal Error
The server could not fulfill the request due to some unexpected condition.:§21.5.1
501 Not Implemented
The server does not have the ability to fulfill the request, such as because it does not recognize the request method. (Compare with 405 Method Not Allowed, where the server recognizes the method but does not allow or support it.):§21.5.2
502 Bad Gateway
The server is acting as a gateway or proxy, and received an invalid response from a downstream server while attempting to fulfill the request.:§21.5.3
503 Service Unavailable
The server is undergoing maintenance or is temporarily overloaded and so cannot process the request. A "Retry-After" header field may specify when the client may reattempt its request.:§21.5.4
504 Server Time-out
The server attempted to access another server in attempting to process the request, and did not receive a prompt response.:§21.5.5
505 Version Not Supported
The SIP protocol version in the request is not supported by the server.:§21.5.6
513 Message Too Large
The request message length is longer than the server can process.:§21.5.7
580 Precondition Failure
The server is unable or unwilling to meet some constraints specified in the offer.
6xx—Global Failure Responses
600 Busy Everywhere
All possible destinations are busy. Unlike the 486 response, this response indicates the destination knows there are no alternative destinations (such as a voicemail server) able to accept the call.:§21.6.1
The destination does not wish to participate in the call, or cannot do so, and additionally the destination knows there are no alternative destinations (such as a voicemail server) willing to accept the call.:§21.6.2
604 Does Not Exist Anywhere
The server has authoritative information that the requested user does not exist anywhere.:§21.6.3
606 Not Acceptable
The user's agent was contacted successfully but some aspects of the session description such as the requested media, bandwidth, or addressing style were not acceptable.:§21.6.4