The Session Initiation Protocol (SIP) is a signalling protocol used for controlling communication sessions such as Voice over IP telephone calls. SIP is based around request/response transactions, in a similar manner to the Hypertext Transfer Protocol (HTTP). Each transaction consists of a SIP request (which will be one of several request methods), and at least one response.
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. A single user agent may act as both UAC and UAS for different transactions: 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) .
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.The SIP response codes are consistent with the HTTP response codes, although not all HTTP response codes are valid in SIP .
SIP responses also specify a "reason phrase", and a default reason phrase is defined with each response code. 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.
The SIP response codes and corresponding reason phrases were initially defined in RFC 3261. That RFC also defines a SIP Parameters Internet Assigned Numbers Authority (IANA) registry to allow other RFC to provide more response codes.
This list includes all the SIP response codes defined in IETF RFCs and registered in the SIP Parameters IANA registry as of 14 July 2017. This list 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.
List of SIP status codes
1xx – Provisional
Preliminary information that the server is still performing further actions and therefore cannot yet send a final response.
2xx – Successful
The request was successful.
3xx – Redirection
These messages inform about a new contact address of the called party or about other services that enable the connection to be established successfully.
4xx – Request Failures
Request failures are negative confirmations. The previous message could not be processed.
5xx – Server Failures
A server involved in the transmission could not process a message.
6xx – Global Failures
General errors: The server has been contacted successfully, but the transaction does not take place.
7xx – Fehlercodes des SIP-Stacks
|100||Trying||An attempt is made to transfer the call.|
|180||Ringing||An attempt is made to ring from the called party.|
|181||Call Is Being Forwarded||The call is forwarded.|
The call is on hold.
|183||Session Progress||The connection is established.|
|199||Early Dialog Terminated||The dialog was closed during connection setup.|
|200||OK||The request has been processed successfully and the result of the request is transferred in the response.|
|202||Accepted||The request has been accepted, but will be executed at a later time.|
|204||No Notification||The request was executed successfully, but the corresponding response is deliberately not sent.|
|300||Multiple Choices||There is no unique destination address for the remote terminal.|
|301||Moved Permanently||The called party is permanently reachable somewhere else.|
|302||Moved Temporarily||The called party is temporarily reachable somewhere else.|
|305||Use Proxy||The specified proxy must be used.|
|380||Alternative Service||The call was not successful, but alternative services are available.|
|400||Bad Request||The SIP request is incorrect.|
|401||Unauthorized||The authorization is incorrect.|
|402||Payment Required||Not yet defined; intended for "not enough credit available".|
|403||Forbidden||The request was invalid.|
|404||Not Found||The remote terminal was not found or does not exist.|
|405||Method Not Allowed||The method of the request (for example, SUBSCRIBE or NOTIFY) is not allowed.|
|406||Not Acceptable||The call options are not allowed.|
|407||Proxy Authentication Required||The proxy needs authorization.|
|408||Request Timeout||Timeout - The remote peer does not respond within a reasonable time.|
|410||Gone||The desired subscriber can no longer be reached at the specified address.|
|412||Conditional Request Failed||The prerequisites for processing the request could not be met because a request required for this failed.|
|413||Request Entity Too Large||The message content is too large.|
|414||Request URI Too Long||The SIP address (URI) of the request is too long.|
|415||Unsupported Media Type||The codec is not supported.|
|416||Unsupported URI Scheme||The SIP address is incorrect.|
|417||Unknown Resource-Priority||The request should be treated with a certain priority, but the server does not understand the details.|
|420||Bad Extension||The server does not understand a protocol extension.|
|421||Extension Required||The server needs a protocol extension.|
|422||Session Interval Too Small||The Session Expires value is too low for the server.|
|423||Interval Too Brief||The value of the desired machining time is too short.|
|428||Use Identity Header||The identity header is missing.|
|429||Provide Referrer Identity||No valid referred by token is specified.|
|430||Flow Failed||The particular routing failed (proxy internal, endpoints should treat the response like code 400).|
|433||Anonymity Disallowed||The server refuses to process anonymous requests.|
|436||Bad Identity-Info||The SIP address contained in the identity header is invalid, unavailable, or not supported.|
|437||Unsupported Certificate||The verifier cannot verify the certificate in the identity header.|
|438||Invalid Identity Header||The certificate in the identity header is invalid.|
|439||First Hop Lacks Outbound Support||The registrar supports outbound feature, but the proxy used does not.|
|440||Max-Breadth Exceeded||It is no longer possible to derive concurrent forks from the query.|
|469||Bad Info Package||Unsuitable Info-Package - Transmission error, resend.|
|470||Consent Needed||The server has no access rights to at least one of the specified SIP addresses.|
|480||Temporarily Unavailable||The called party is currently not reachable.|
|481||Call/Transaction Does Not Exist||This connection does not exist (anymore).|
|482||Loop Detected||A forwarding loop has been detected.|
|483||Too Many Hops||Too many forwarding steps were identified.|
|484||Address Incomplete||The SIP address is incomplete.|
|485||Ambiguous||The SIP address cannot be uniquely resolved.|
|486||Busy Here||The called party is busy.|
|487||Request Terminated||The call attempt was aborted.|
|488||Not Acceptable Here||Illegal call attempt.|
|489||Bad Event||The server does not know the specified event.|
|491||Request Pending||A request from the same dialog is still being processed.|
|493||Undecipherable||The request contains an encrypted MIME body that the recipient cannot decrypt.|
|494||Security Agreement Required||The request requires a security agreement, but does not include a security mechanism supported by the server.|
|500||Server Internal Error||Internal server error.|
|501||Not Implemented||The server does not support the SIP request.|
|502||Bad Gateway||The gateway in the SIP request is corrupted.|
|503||Service Unavailable||The server's SIP service is temporarily unavailable.|
|504||Server Time-out||The server cannot reach another server in a reasonable time.|
|505||Version Not Supported||The SIP protocol version is not supported by the server.|
|513||Message Too Large||The SIP message is too large for UDP; TCP must be used.|
|580||Precondition Failure||The server cannot or does not want to meet the requirements for processing the request.|
|600||Busy Everywhere||All terminal devices of the called subscriber are occupied.|
|603||Declined||The called party has rejected the call attempt.|
|604||Does Not Exist Anywhere||The called party no longer exists.|
|606||Not Acceptable||The terminal device of the called party rejects the SIP request as invalid.|
|701||Party Hangs Up||The called party has hung up.|