Locate the response family
- 1xx ProvisionalRequest is progressing
- 2xx SuccessRequest succeeded
- 3xx RedirectionAnother destination
- 4xx Request failureInterpret this request488 belongs here
- 5xx Server failureServer could not fulfil
- 6xx Global failureFailure across destinations
Read the status with its request method and call leg. For example, 200 after REGISTER confirms a binding; it does not prove an INVITE succeeded.
What this response means
The proposed session parameters cannot be accepted at this destination. In voice deployments, media negotiation is a useful first area to investigate.
Read it in context
Codec overlap, SDP structure, media-security policy or another session constraint can be involved. It is not safe to conclude that one particular codec is always missing from the code alone.
Administrator checks
Compare offer and answer-related details on the rejecting leg. Review accepted audio formats, RTP event support, encryption requirements and any diagnostic headers or logs. A PBX can negotiate different formats on each side, so inspect both legs.
Next action and common mistake
Do not enable every codec or disable SRTP indiscriminately. Establish a documented compatible policy and test incoming, outgoing and transferred calls. Confirm actual negotiated media and sound after setup succeeds.
Sources & applicability
Primary references for the technical details above. Operational examples and planning checklists are VoIP.info editorial guidance.