Source: https://www.3cx.com/PBX/SIP-responses/

What are SIP responses?

SIP responses are the codes used by Session Initiation Protocol for communication. We have put together a list of all the SIP responses known.

1xx = Informational SIP responses

• 100 Trying – Extended search is being performed so a forking proxy must send a 100 Trying response.
• 180 Ringing – The Destination User Agent has received the INVITE message and is alerting the user of call.
• 181 Call Is Being Forwarded – Optional, send by Server to indicate a call is being forwarded.
• 182 Queued – Destination was temporarily unavailable, the server has queued the call until the destination is available.
• 183 Session Progress – This response may be used to send extra information for a call which is still being set up.
• 199 Early Dialog Terminated – Send by the User Agent Server to indicate that an early dialogue has been terminated.

2xx = Success responses

• 200 OK – Shows that the request was successful.
• 202 Accepted – Indicates that the request has been accepted for processing, mainly used for referrals.
• 204 No Notification – Indicates that the request was successful but no response will be received.

3xx = Redirection responses

• 300 Multiple Choices –  The address resolved to one of several options for the user or client to choose between.
• 301 Moved Permanently – The original Request URI is no longer valid, the new address is given in the Contact header.
• 302 Moved Temporarily – The client should try at the address in the Contact field.
• 305 Use Proxy – The Contact field details a proxy that must be used to access the requested destination.
• 380 Alternative Service – The call failed, but alternatives are detailed in the message body.

4xx = Request failures

• 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 (User not found).
• 405 Method Not Allowed – The method specified in the Request-Line is understood, but not allowed.
• 406 Not Acceptable – The resource is only capable of generating responses with unacceptable content.
• 407 Proxy Authentication Required – The request requires user authentication.
• 408 Request Timeout – Couldn’t find the user in time.
• 409 Conflict – User already registered (deprecated).
• 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 (deprecated).
• 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 – Server refuses to service the request, the Req-URI is longer than the server can interpret.
• 415 Unsupported Media Type – Request body is in a non-supported format.
• 416 Unsupported URI Scheme – Request-URI is unknown to the server.
• 417 Uknown 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 request contains a Session-Expires header field with a duration below the minimum.
• 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.
• 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 contained cannot be de-referenced.
• 437 Unsupported Certificate – The server was unable to validate a certificate for the domain that signed the request.
• 438 Invalid Identity Header – Server obtained a valid certificate used to sign a request, was unable to verify the signature.
• 439 First Hop Lacks Outbound Support – The first outbound proxy doesn’t support the “outbound” feature.
• 440 Max-Breadth Exceeded – If a SIP proxy determined a response context had insufficient Incoming Max-Breadth to carry out a desired parallel fork, and the proxy is unwilling/unable to compensate by forking serially or sending a redirect, that proxy MUST return a 440 response. A client receiving a 440 response can infer that its request did not reach all possible destinations.
• 469 Bad Info Package – If a SIP UA receives an INFO request associated with an Info Package that the UA has not indicated willingness to receive, the UA MUST send a 469 response, which contains a Recv-Info header field with Info Packages for which UA is willing to receive INFO requests.
• 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 dialogue 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 aspects of the session description of the Request-URI are 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 dialogue.
• 493 Undecipherable – UndecipherableRequest 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.

5xx = Server errors

• 500 Server Internal Error – The server could not fulfill the request due to some unexpected condition.
• 501 Not Implemented – The SIP request method is not implemented here.
• 502 Bad Gateway – The server, received an invalid response from a downstream server while trying to fulfill a request.
• 503 Service Unavailable – The server is in maintenance or is temporarily overloaded and cannot process the request.
• 504 Server Time-out – The server tried to access another server while trying to process a request, no timely response.
• 505 Version Not Supported – The SIP protocol version in the request is not supported by the server.
• 513 Message Too Large – The request message length is longer than the server can process.
• 555 Push Notification Service Not Supported – The server does not support the push notification serviced specified in the pn-provider SIP URI parameter.
• 580 Precondition Failure – The server is unable or unwilling to meet some constraints specified in the offer.

6xx = Global failures

• 600 Busy Everywhere – All possible destinations are busy.
• 603 Decline – Destination cannot/doen’t wish to participate in the call,  no alternative destinations.
• 604 Does Not Exist Anywhere – The server has authoritative information that the requested user does not exist anywhere.
• 606 Not Acceptable – The user’s agent was contacted successfully but some aspects of the session description were not acceptable.
• 607 Unwanted – The called party did not want his call from the calling party. Future attempts from the calling party are likely to be similarly rejected.