You just need to activate (and jailbreak) the iphone using tools already available. You don't need to activate it through at&t.
If Tmobile supports EDGE and you want to have data on the go then you need to contact tmobile and see how much the data plans cost. As long as your mobile provider supports EDGE, you will be able to use it using this hack.
Visual voice mail will not work. You will need to call your mailbox just like you did before the iPhone existed.
For this software unlock you will need to have your phone activated and jailbreaked. Google for more info regarding these two.