People ask me if such a thing is ever going to happen and for now I think its kind of hard to tell. As with a lot of stuff in Unified Communications, its not only about having the technology in place to make the changes but also about changing the way in which people work and use the technology made available to them. You can give people the ability to check someone's presence before making a phone call but if they don't remember or learn to check it themselves then they wont see the benefits.
As mentioned in my last blog posting, there are a number of people 'dogfooding' Office Communications Server 2007 and Communicator 2007 within Microsoft and a few of them have made the jump to remove their deskphone totally!. Take a look here where Mu Han talks about his experiences so far as well as some of the technology that has gone into Communicator, OCS and even Windows Live Messenger to make this possible ... it actually a really interesting read into what the future may hold and should give you an idea of how soon its coming !