This is exactly the space that companies coming onto the scene are playing, especially Cisco, which a few years ago announced integration alliance with the traditional telco giant SBC which in effect was designed to move the traffic from circuit switched to IP.
But Cisco isn't loyal, and if a customer decides to move from SBC to Level3 or Sprint as their network provider, they don't exactly yank out the routers and hubs. Or do they?
All of that said, the space is changing. With new companies like Stealth Communications (they help power Vonage) coming on the scene and becoming the glue to make things happen solutions that work are being linked together by the internet allowing enterprise customers to say bye bye to the Bells.