SIP error reference
Quick reference for the SIP response codes you'll actually see in production, with vendor-specific notes for Asterisk, FreeSWITCH, Kamailio, Cisco CUBE, and friends.
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SIP 401 Unauthorized
RFC 3261 §22.1The server is challenging the request for digest authentication. This is normal on the first INVITE / REGISTER and almost never indicates an issue on its own — but a *loop* of 401s usually does.
SIP 403 Forbidden
RFC 3261 §21.4.4The server understood the request but refuses to authorize it. Often caused by IP ACLs, blocked numbers, or trunk billing checks.
SIP 408 Request Timeout
RFC 3261 §21.4.9The server didn't get a response within the SIP transaction timeout (Timer B = 32s for INVITE). Usually a routing or downstream issue.
SIP 480 Temporarily Unavailable
RFC 3261 §21.4.18Callee is registered but not in a state to accept the call (DND, forwarded, no endpoint listening, etc.).
SIP 486 Busy Here
RFC 3261 §21.4.21The endpoint is busy on another call. Distinct from 600 Busy Everywhere.
SIP 488 Not Acceptable Here
RFC 3261 §21.4.26The destination accepted the request but cannot honour the SDP offer. The classic codec-mismatch error: the offered codecs don't intersect the callee's allow-list.
SIP 491 Request Pending
RFC 3261 §14.1A re-INVITE collided with a pending request. RFC 3261 prescribes a backoff retry; usually transient.
SIP 503 Service Unavailable
RFC 3261 §21.5.4The server is temporarily unable to process the request. Often emitted by SBCs / proxies under load or when the next-hop is down.