Apple gets a share of the monthly fee the end user pays to AT&T.
Why? I'm sorry but this seems pretty damn greedy to me. They're already making 50% profit on the hardware itself (fair enough, they make it), but they also want a cut of your contract fee because... what?
Are they going to ask for a cut of your internet service provider's fee next if you use a Mac on the internet?
No wonder other carriers turned them down, no other hardware manufacturer would have the balls to ask for that. More fool to AT&T for agreeing to such ridiculous terms.
Have you really seen usage statistics of the first four days, broken down to voicemail use, and customer satisfaction with the new system?
I guess not.
Like I said, it's hardly worth locking out 80% of your market for this feature. It's a minimal impact convenience feature at best. People can still use voicemail without it, you know.
They've cut out the vast majority of their potential market by locking the iPhone down to one carrier.