SIP Fundamentals

SIP Response Codes (1xx-6xx)

SIP response codes are 3-digit status codes patterned after HTTP. The first digit indicates the class: 1xx provisional, 2xx success, 3xx redirection, 4xx client error, 5xx server error, 6xx global failure.

Cheat-sheet of the codes you will actually see

Code	Meaning	What to do
100 Trying	Provisional — request received	Wait for next response
180 Ringing	Callee is being alerted	Show ringback to caller
183 Session Progress	Early media (audio before answer)	Connect early media
200 OK	Success — call answered	Send ACK, start RTP
301 Moved Permanently	Use new SIP URI	Redirect
302 Moved Temporarily	Use new URI for this call only	Redirect
400 Bad Request	Malformed SIP message	Fix request format
401 Unauthorized	Auth required	Re-send with Authorization header
403 Forbidden	Authenticated but not authorized	Check permissions / ACL
404 Not Found	Number / URI does not exist	Check destination
407 Proxy Auth Required	Proxy needs auth	Re-send with Proxy-Authorization
408 Request Timeout	No response from upstream	Retry / failover
480 Temporarily Unavailable	Callee not reachable	Try voicemail / next route
486 Busy Here	Callee is busy	Try forwarding
487 Request Terminated	Caller hung up before answer	Normal cancellation
488 Not Acceptable Here	Codec / SDP mismatch	Renegotiate codec
500 Server Internal Error	Carrier internal failure	Retry / report
503 Service Unavailable	Upstream overloaded	Failover to secondary
504 Gateway Timeout	Upstream did not respond	Failover
603 Decline	Callee declined	End

Mapping to PSTN cause codes

SIP response codes map to ISDN Q.850 cause codes via RFC 3398. Q.850 17 (User busy) ↔ SIP 486; Q.850 16 (Normal clearing) ↔ SIP BYE with no special code; Q.850 21 (Call rejected) ↔ SIP 603. Carriers often add a Reason: Q.850;cause=21 header so you can see the underlying PSTN reason.

Related terms