A SIP response is a message generated by a user agent server (UAS) or SIP server to reply to a request generated by a client.
A response may contain some additional header fields of info needed by a UAC.
SIP has six responses.
1xx to 5xx has been borrowed from HTTP and 6xx is introduced in SIP.
1xx is considered as a provisional response and the rest are final responses.
SIP Error starting with 1xx—Provisional Responses
100 Trying
Extended search being performed may take a significant time so a forking proxy must send a 100 Trying response.[1]:§21.1.1
180 Ringing
Destination user agent received INVITE, and is alerting user of call.[1]:§21.1.2
181 Call is Being Forwarded
Servers can optionally send this response to indicate a call is being forwarded.[1]:§21.1.3
182 Queued
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.[1]:§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.[1]:§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.[4]
SIP Error starting with 2xx—Successful Responses
200 OK
Indicates the request was successful.[1]:§21.2.1
202 Accepted
Indicates that the request has been accepted for processing, but the processing has not been completed.[5]:§7.3.1[6] Deprecated.[7]:§8.3.1[3]
204 No Notification
Indicates the request was successful, but the corresponding response will not be received.[8]
SIP Error starting with 3xx—Redirection Responses
300 Multiple Choices
The address resolved to one of several options for the user or client to choose between, which are listed in the message body or the message's Contact fields.[1]:§21.3.1
301 Moved Permanently
The original Request-URI is no longer valid, the new address is given in the Contact header field, and the client should update any records of the original Request-URI with the new value.[1]:§21.3.2
302 Moved Temporarily
The client should try at the address in the Contact field. If an Expires field is present, the client may cache the result for that period of time.[1]:§21.3.3
305 Use Proxy
The Contact field details a proxy that must be used to access the requested destination.[1]:§21.3.4
380 Alternative Service
The call failed, but alternatives are detailed in the message body.[1]:§21.3.5
SIP Error starting with 4xx—Client Failure Responses
400 Bad Request
The request could not be understood due to malformed syntax.[1]:§21.4.1
401 Unauthorized
The request requires user authentication. This response is issued by UASs and registrars.[1]:§21.4.2
402 Payment Required
Reserved for future use.[1]:§21.4.3
403 Forbidden
The server understood the request but is refusing to fulfill it.[1]:§21.4.4 Sometimes (but not always) this means the call has been rejected by the receiver.
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.[1]:§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.[1]:§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.[1]:§21.4.7
407 Proxy Authentication Required
The request requires user authentication. This response is issued by proxies.[1]:§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. [1]:§21.4.9
409 Conflict
User already registered.[9]:§7.4.10 Deprecated by omission from later RFCs[1] and by non-registration with the IANA.[3]
410 Gone
The user existed once but is not available here anymore.[1]:§21.4.10
411 Length Required
The server will not accept the request without a valid Content-Length.[9]:§7.4.12 Deprecated by omission from later RFCs[1] and by non-registration with the IANA.[3]
412 Conditional Request Failed
The given precondition has not been met.[10]
413 Request Entity Too Large
Request body too large.[1]:§21.4.11
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.[1]:§21.4.12
415 Unsupported Media Type
Request body in a format not supported.[1]:§21.4.13
416 Unsupported URI Scheme
Request-URI is unknown to the server.[1]:§21.4.14
417 Unknown Resource-Priority
There was a resource-priority option tag, but no Resource-Priority header.[11]
420 Bad Extension
Bad SIP Protocol Extension used, not understood by the server.[1]:§21.4.15
421 Extension Required
The server needs a specific extension not listed in the Supported header.[1]:§21.4.16
422 Session Interval Too Small
The received request contains a Session-Expires header field with a duration below the minimum timer.[12]
423 Interval Too Brief
The expiration time of the resource is too short.[1]:§21.4.17
424 Bad Location Information
The request's location content was malformed or otherwise unsatisfactory.[13]
428 Use Identity Header
The server policy requires an Identity header, and one has not been provided.[14]:p11
429 Provide Referrer Identity
The server did not receive a valid Referred-By token on the request.[15]
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).[16]:§11.5
433 Anonymity Disallowed
The request has been rejected because it was anonymous.[17]
436 Bad Identity-Info
The request has an Identity-Info header, and the URI scheme in that header cannot be dereferenced.[14]:p11
437 Unsupported Certificate
The server was unable to validate a certificate for the domain that signed the request.[14]: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.[14]: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.[16]:§11.6
470 Consent Needed
The source of the request did not have the permission of the recipient to make such a request.[18]
480 Temporarily Unavailable
Callee is currently unavailable.[1]:§21.4.18
481 Call/Transaction Does Not Exist
The server received a request that does not match any dialog or transaction.[1]:§21.4.19
482 Loop Detected.
The server has detected a loop.[1]:§21.4.20
483 Too Many Hops
Max-Forwards header has reached the value '0'.[1]:§21.4.21
484 Address Incomplete
Request-URI incomplete.[1]:§21.4.22
485 Ambiguous
Request-URI is ambiguous.[1]:§21.4.23
486 Busy Here
Callee is busy.[1]:§21.4.24
487 Request Terminated
The request has terminated by bye or cancel.[1]:§21.4.25
488 Not Acceptable Here
Some aspect of the session description or the Request-URI is not acceptable.[1]:§21.4.26
489 Bad Event
The server did not understand an event package specified in an Event header field.[5]:§7.3.2[7]:§8.3.2
491 Request Pending
The server has some pending requests from the same dialog.[1]:§21.4.27
493 Undecipherable
The request contains an encrypted MIME body, which recipient cannot decrypt.[1]:§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,[19]:§§2.3.1–2.3.2 or a digest authentication challenge.[19]:§2.4
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SIP Error starting with 5xx—Server Failure Responses
500 Server Internal Error
The server could not fulfill the request due to some unexpected conditions.[1]:§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.)[1]:§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.[1]:§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.[1]:§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.[1]:§21.5.5
505 Version Not Supported
The SIP protocol version in the request is not supported by the server.[1]:§21.5.6
513 Message Too Large
The request message length is longer than the server can process.[1]:§21.5.7
580 Precondition Failure
The server is unable or unwilling to meet some constraints specified in the offer.[20]
SIP Error starting with 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.[1]:§21.6.1
603 Decline
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.[1]:§21.6.2
604 Does Not Exist Anywhere
The server has authoritative information that the requested user does not exist anywhere.[1]:§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.[1]:§21.6.4