1xx – Provisional Responses (Informational SIP Codes)

1xx SIP Provisional Response codes are sent while a SIP session is being established.

A server will send a 1xx response if it is likely to take more than 200ms to receive a final response. This indicates that the server being contacted requires more time and further action to give a final response to the originating server.

It is important to note that these responses are often not transmitted reliably.

100 Trying – The connection request has been received by the Next Hop server and extended search is being performed.

180 Ringing – A 180 Ringing response indicates that the destination UA (User Agent) has received the INVITE.

181 Call is Being Forwarded – 181 Response indicates that the call is being forwarded to a different destination. This response may or may not be sent.

182 Queued – The destination was temporarily unavailable, and the caller has been placed in a queue. The server may send multiple 182 responses as status updates on the queue.

183 Session Progress – 183 Response provides information about a call while it is being set up.

199 Early Dialog Terminated

2xx – Success SIP Response Codes

2xx Responses indicate that a connection has been successful.

200 OK – The connection request has been successful. Further information may be included depending on how the request was made.

202 Accepted – This response code has been depreciated, however a 202 Response indicated that a request had been accepted for processing and the processing was in progress.

204 No Response – A 204 Response indicates that the request was successful, however no response will be received.

3xx – Redirection Responses

3xx Responses indicate a redirection of the request.

300 Multiple Choices – The address in the request resolved to a number of choices for the user or client to choose between. The locations are listed in the message body or Contact fields.

301 Moved Permanently – The user is no longer accessible via the address in the request. The new address is available in the Contact header field.

302 Moved Temporarily – The user is not available via the address in the request for a short amount of time. The request should be retried with the new address supplied in the Contact header field.

305 Use Proxy – A proxy must be used to reach the requested destination. The proxy is detailed in the Contact field.

380 Alternative Service – A 380 response indicates that the request was not successful, however alternative services are possible. The services are available in the message body of the response.

4xx – Request Failure Responses (SIP Error Codes)

4xx SIP response codes indicate that the request to a particular server has failed.

400 Bad Request – The 400 Bad Request response will be returned when there is a syntax error in the request. The syntax error is often identified by the Reason-Phrase in greater detail.

401 Unauthorised – Authentication is required for the request.

402 Payment Required – Reserved for future use.

403 Forbidden – The request was understood, but the server refused to accept it. The request should not be repeated.

404 Not Found – The address/ user requested does not exist.

405 Method Not Allowed – The method of the request is understood but not allowed by the destination address.

406 Not Acceptable – The resource identified in the request is only able to generate responses with content unacceptable to the originating server.

407 Proxy Authentication Required – The 407 response is similar to 401, but this response requires authentication with the proxy.

408 Request Timeout – The server was not able to find the user within a suitable amount of time. This request may be repeated at any time.

409 Conflict – This response has been depreciated, however was used to indicate that a user was already registered.

410 Gone – This response indicates that while the user being contacted existed once, they are no longer available.

411 Length Required – This response has been depreciated, however was used to indicate that the server will not accept a request without a valid content length.

413 Request Entity Too Large – The body of the request is too large.

414 Request URI Too Long – This response indicates that the server has refused the request as the Req-URI is longer than can be interpreted.

415 Unsupported Media Type – The 415 response indicates that the body of the request is in a non-supported format.


416 Unsupported URI Scheme – The Req-URI is unknown to the server.

417 Unknown Resource-Priority – The 417 response is sent when there was a resource-priority option tag, but no Resource-Priority header was found.

420 Bad Extension – The SIP Protocol Extension used was not understood by the server.

421 Extension Required – The extension required by the server was not listed in the Supported header.

422 Session Interval Too Small – The Session-Expires duration of the request was below the minimum required.

423 Interval Too Brief – Response 423 indicates that expiration time of the resource is too short.

424 Bad Location Information – The request’s location content was incorrect or otherwise unsatisfactory.

428 Use Identity Header – This response indicates that the server policy requires an Identity header and there was not one supplied in the request.

429 Provide Referrer Identity – The 429 response is intended for use between proxy devices. It indicates that the server did not receive a Referred-By token in the request.

433 Anonymity Disallowed – 433 Response code indicates that the request has been rejected because it was anonymous.

436 Bad Identity-Info – An Identity-Info header was included in the request and the URI scheme contained cannot be dereferenced.

437 Unsupported Certificate – This response indicates that the certificate for the domain that signed the request was unable to be validated by the server.

438 Invalid Identity Header – The certificate obtained by the server was valid, however the signature was not able to be verified.

439 First Hop Lacks Outbound Support This response indicates that the first outbound proxy doesn’t support an outbound feature.

470 Consent Needed – The source of the request did not have appropriate permissions to make the request.

480 Temporarily Unavailable – SIP response 480 indicates that the request recipient is currently unavailable.

481 Call/ Transaction Does Not Exist – The request received by the server does not match any existing dialog or transaction.


482 Loop Detected – A loop has been detected by the server.

483 Too Many Hops – The Max-Forwards header has reached the value of ‘0’.

484 Address Incomplete – The address requested was incomplete.

485 Ambiguous – The Request-URI was ambiguous.

486 Busy Here – The recipient was contacted successfully, however the recipient was reported busy/ unable to take additional calls.

487 Request Terminated – This response code is sent when a request is terminated by a BYE or CANCEL request.

488 Not Acceptable Here – The SIP error 488 response is the same as a 606 (Not Acceptable) response code, however the 488 response only applies to the specific resource addressed by the Request-URI. The same request may succeed elsewhere.

491 Request Pending – The requested server has pending requests from the same dialog.

493 Undecipherable – The recipient server cannot decrypt the encrypted MIME body contained in the request.

494 Security Agreement Required The 494 response indicates that the server has received a request that requires a negotiated security mechanism. The response will contain a list of security mechanism that the requester is to choose from.

5xx – Server Failure Responses

A 5xx response will be returned when an error has been encountered on a server.

500 Server Internal Error – The server was prevented from fulfilling the request due to an unexpected condition. A specific error condition may be displayed, and the request may be retried after several seconds.

501 Not Implemented – The 501 error response is returned when the functionality required to fulfil the request is not supported by the server.

502 Bad Gateway – An invalid response was received by the server from a downstream server while trying to fulfil a request.

503 Service Unavailable – SIP response 503 indicates that the server is temporarily unavailable, usually due to maintenance or a temporary overload.

504 Server Time-Out – There was no timely response from the other server while trying to process a request.

505 Version Not Supported – The SIP protocol version in the request is not supported by the server.

513 Message Too Large – The server is not able to process the request due to the message length.

580 Precondition Failure – The server is unable or unwilling to meet some constraints in the request.

6xx – Global Failure Responses

6xx responses return information about a user, rather than just the particular instance.

600 Busy Everywhere – The request recipient was contacted successfully, however the recipient is unable to take the call. This response is only retuned if there is no other end point to answer the request, such as a voicemail. If there is another end point, the request will return a 486 response instead.

603 Decline – The request recipient was contacted successfully, however the request was declined. This response is only returned if there is no other end point to answer the request.

604 Does Not Exist Anywhere – The user requested does not exist anywhere.

Aatrox Communications is an Australian owned and operated full service SIP provider. We specialise in connecting the traditional telephony network to the internet for small and large businesses.