How many phones can fit on the head of a pin
I don't foresee this happening anytime soon, at least not in the US.
Why? Simple: redundancy.
I worked in the telecommunications dept of a multinational corporation, specifically working with the branches that are stateside. They had between 100 and 120 branches of varying sizes, some of which were little sales offices, some ranging up to a couple hundred lines into the single building. What's the one thing they had in common? Every single branch has a copper main number. Drove the carriers nuts. "Won't you consolidate your main line into a T1? It's cheaper and easier to manage that way for both of us!" Never happened, and I seriously doubt it will happen anytime in the next 20 years.
The reason for that is that, in the event of, say, a power outage, some informed person with a backup plan document can plug their analog phone into the copper line, and boom, you can talk to wherever is needed. Those T1 lines? Good luck with keeping those up, especially after your UPS dies. Are you going to honestly tell me that my computer can give me the same sort of functionality as an old analog, 4 wire phone?
I mean, hell, when I was working there, I had three different phones on my desk, each with a valid specific purpose. Almost everyone in the department had 3, one of which was always a copper line.
A second note: Cost. How many big companies are going to shell out for the hardware for setting all of this up, both on the user side and back end? Hell, of those ~100 branches, about 30 were still running Executone phone systems. Reliable, maybe, but the end range for the copyright date you see when you log in on those systems is 1992. Most large companies are under the impression of "If it ain't broke, don't fix it."