It's been a few years since I worked in softphones, but this "problem" is essentially solved with the SIP protocol. Everyone gets an address of the form user@domain. (The protocol scheme is sip: but the beauty of using user@domain is that one can quickly switch to mailto:). Then the immediacy of the communication dictates the medium. Don't need an answer right away? Email. Low priority? IM/Text. Must talk now? Voice.

I believe Office Communicator (or whatever it's called now) was designed with this in mind.

But that doesn't negate the need for an address book. One still needs to know the user@domain address, as others have pointed out.