488 4xx Client error RFC 3261
SIP 488 Not Acceptable Here
Some aspect of the session description (SDP) was not acceptable, most often no common codec.
Typical Q.850 / ISDN cause: 58 (Bearer capability not presently available)
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Common causes
- No codec in common (for example, caller offers only Opus or G.722 and the trunk accepts only G.711 or G.729).
- SRTP required on one side but not offered (or the reverse).
- Wrong ptime, unsupported DTMF payload, or T.38 versus G.711 fax mismatch.
How to fix it
- Compare the m= lines in the INVITE's SDP with the codecs the far end allows. Enable a shared codec such as G.711 A-law (PCMA).
- Align the SRTP and encryption settings on both sides.
- Allow transcoding on your SBC or PBX if you cannot share a codec.
Seeing 488 on calls through a router or firewall? Check that SIP ALG is disabled. It causes a surprising number of SIP errors. Read why.
About 4xx responses
The request has a problem or cannot be completed at this server, usually because of something on the requesting side or the target user.