I have it fully working now with voipover3g, however when I try to make a call I get a called failed message. Same call works fine over wi-fi. Maybe t-mobile is blocking it on their side, would that be possible?
Make sure you are typing a 1 before the area code (for example, +1555-555-1212). I was getting the call failed message myself until I realized you have to put a 1 in front of the area code. It's really annoying too because none of my contacts' numbers have a 1 before the area code in my contacts app. Hopefully they come up with a solution to that problem.