Yes data and voice can be used on TMobile. I think even on the pre-paid network too. The reason why they want you to pay after and not in front in case you incur any roaming charges. If you prepay then they can't get you to pay roaming charges if you do roam (thus their solution of just not giving you service if your out of the network area).

But if your paying monthly (or once you get your bill) then they let you roam if you want to since they know they can get you to pay it.