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Collaboration, Voice & AV · Intermediate

SIP Response Codes

Three-digit SIP status codes grouped into classes 1xx provisional, 2xx success, 3xx redirection, 4xx client error, 5xx server and 6xx global failure.

Definition

SIP borrows HTTP-style status codes. Common examples include 100 Trying, 180 Ringing, 183 Session Progress, 200 OK, 302 Moved Temporarily, 401 Unauthorized, 403 Forbidden, 404 Not Found, 407 Proxy Authentication Required, 408 Request Timeout, 486 Busy Here, 488 Not Acceptable Here, 503 Service Unavailable and 603 Decline. Reading these codes in traces is the main SIP troubleshooting skill.

Example

A carrier returns 488 Not Acceptable Here because the INVITE offered only a codec the carrier does not support.

Exam tip

488 usually signals a codec or SDP mismatch, 503 a downstream resource or trunk problem, and 404 a dial plan or number issue.

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