SIP
606 Not Acceptable
The user's agent was contacted successfully but some aspects of the session description (SDP) are not acceptable. Unlike 488, this is a global failure — no other server can satisfy this request.
When You See This Code
When the callee definitively cannot accept any of the offered media types or codecs. Different from 488 which is location-specific.
How to Fix
Check the Warning header or response body for which session parameters are unacceptable. Modify the SDP offer to use compatible codecs and media types.