It won't work. Some read up from another site:
Apple used the Inferion BGA736, a 3 Band (800-900)/1900/2100MHz chip.
It is missing the critical 1700MHz band that would allow it to work universally on the U.S. carriers 3G networks. So you will be able to use the 3G portion only on ATT, and in Europe. But that would not stop you from cracking the 3G iPhone and using only EDGE on T-mobiles network becuase it would still need EDGE radio to fall back onto on ATTs sparse 3G network.
As for visual voicemail, I doubt seriously that you will see it working because this requires an upgrade on the carriers and of things. And why would T-Mobile spend money to support a feature on a phone they technically dont support. Now T-mobile in Europe does sell the iPhone, and has visual voicemail but again why spend the money here.