I was interested by this new IETF Working Group mentioned on in Dan York's blog. BLISS, or Basic Level of Interoperability for SIP Services is setting out to define a set of common SIP functions that should interoperate between different vendors phones and switches, including: Bridges/Shared Line Appearance; Call Park/Pickup; Do Not Disturb and Call Completion on Busy/No Reply.

As they say in their charter, SIP has a large toolkit of primitive functions that can be combined in various ways to create different service behaviours. Unfortunately, a rich toolbox also means more potential for creating incompatible ways of doing things.

Incompatibility is a huge problem with proprietary PBXes today, so it's good to see the IETF trying to create some standardization before it becomes a huge problem also with SIP systems.