[sip-comm] Compliance to RFC 3312


I'm looking for a Sip User Agent that supports RFC 3312.
What I mean is:
- session's parameters (e.g. codec) negotiaton with SDP exchange
- the called user agent rings only when preconditions are met.
I looked around in sip-communicator faq and mailing list and I've found the
only message about this feature is:


but it's 2 years old. Is this feature still working in progress?
Could you give me some help or suggestions?
Thank you