SIP Response Codes
The RFC tells you 488 means "Not Acceptable Here". It does not tell you that it is almost always a codec mismatch in the SDP answer. This lists the practical cause alongside the specification meaning.
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Voice & VoIP
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Reading a failure
Which end to look at when a call does not complete.
- 4xx — this request was wrong. The same request will fail again; something in the message or the credentials has to change.
- 5xx — this server failed. Another server for the same domain may well succeed, which is why a UA is allowed to try one.
- 6xx — the user is unavailable everywhere, not just at this server. A 603 Decline ends forking; a 486 Busy Here does not. Getting this distinction wrong is why calls sometimes stop hunting to voicemail.