values of SIP Accept and SIP Accept-Contact

A lot of sections in the [SIP RFC]1 are based on the [HTTP 1.1 RFC]2 in acknowledgement that the semantics of SIP and HTTP are very similar. The SIP Accept header is a good case in point. The SIP RFC section that deals with the Accept header refers to [H14.1] which translates to section 14.1 in the HTTP 1.1 RFC and which goes into detail about how the Accept header can be used to specify the different types of media that are acceptable in the response.

That all being said in the real World 90% of the time the SIP response media is going to be application/sdp. There will be SIP requests that accept other types of response media but they are not that common.