This idea came about from thinking about the uses of a voice over IP (VoIP) system at university campuses, especially when married to instant message and presence features. Basically, your "identity" is matched to your "phone number". By default, your identity resides on your phone, in your dorm room. Might even have the situation where multiple "identities" are mapped to the same phone -- perhaps the shared phone in the common area (where 4 students share housing -- not for an entire dorm). Even a dorm room with 2 students could share one phone, but have 2 identities. You might even need to authenticate yourself to use (or answer) calls, although some dorm roomies might say "let identity B [my roommate] answer calls for me". Now, that's just the extrapolation.
My main idea was that you could logon to the presence/voice/multimedia system with a soft client from elsewhere, say from your laptop in the library. Your identity would actually be "logged off" your main phone in your room. Like a regular phone, you could choose to accept calls, or send them to voice mail -- or in the ideal SIP world, accept the call, but try and negotiate an IM session instead (because you're in the library, and can't talk). This functionality is already present in today's IM sessions -- if you log into your account from a 2nd device when you're already logged in elsewhere, the 1st device is kicked off (there is only one "you" at any one time). I haven't read over the relevant RFC's in a while, but the Instant Messaging and Presence Protocol (IMPP) working group is the place to look.