Comment 13 for bug 294991

(In reply to comment #4)
> http://sofia-sip.sourceforge.net/ suggests that the underlying sofiasip library
> supports SIMPLE... what would be needed is an implementation of the MESSAGE,
> SUBSCRIBE/NOTIFY and PUBLISH methods from the respective rfcs (
> http://en.wikipedia.org/wiki/SIMPLE )

True, it's mostly a simple matter of programming.
Another thing is XCAP contact list support, which has some disclosed IPR encumberments. To implement that (and it's the only semi-sane standardized way to manage subscription lists with regard to SIP), one has to start with developing a plugin system to telepathy-sofiasip.