There seem to be some issues with visual voicemail on Windows Phones running 7.5 (Mango) on T-Mobiles network. Odd fix, but the solution: text "Activate:dt=6" to 122 without the quotes. There’s also a way to activate it online but it’s noted that you have to make sure you check ‘start immediately’ and some who have issues think there may be better luck if you have no voicemails pending when you activate it.
One way or another, seems like a silly problem with a silly solution, but at least it has a known solution now.
via Shawn Wildermuth