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SIP response codes: a practical troubleshooting guide
SIP response codes tell you how a SIP server or endpoint handled a request. Start with the code class, then inspect the request, the responding hop, authentication, routing, and the response body.
Common codes
| Code | Meaning | First checks |
|---|---|---|
| 100 | Trying | Request reached a SIP element; wait for the next response |
| 180 | Ringing | Destination is being alerted |
| 183 | Session Progress | Early media or provisional progress; inspect SDP |
| 200 | OK | Request succeeded; confirm the ACK and media path |
| 401 | Unauthorized | Digest authentication challenge; verify credentials and realm |
| 403 | Forbidden | Request understood but refused; check permissions, identity, and policy |
| 404 | Not Found | User, route, or domain was not found |
| 408 | Request Timeout | No timely response; check reachability and upstream routing |
| 480 | Temporarily Unavailable | Endpoint unavailable, busy, offline, or policy-limited |
| 486 | Busy Here | Called endpoint is busy or rejects the call |
| 487 | Request Terminated | An earlier request was cancelled, often after CANCEL |
| 488 | Not Acceptable Here | SDP or media offer is not acceptable |
| 500 | Server Internal Error | Server-side failure; inspect logs and upstream response |
| 503 | Service Unavailable | Capacity, maintenance, overload, or unavailable route |
| 603 | Decline | Destination explicitly declined the call |
The important distinction
The response code describes signalling, not necessarily the final user experience. A call can receive 200 OK and still have silence if RTP is blocked, advertised with the wrong address, or negotiated with an unusable codec.
FAQ
Is 401 always an error?
No. A server commonly uses 401 to request digest authentication; the client should retry with credentials. Repeated 401 responses usually point to credentials, realm, nonce handling, or account configuration.
What should I do with a 488?
Compare the SDP offer and answer. Look for no common codec, unsupported encryption, incompatible media direction, or an unreachable media profile.
Reference: RFC 3261 SIP.