Big iPhone problems 3.1.2 FM

[bmwmon]

I am have troubles with my unlocked(blackra1n) iphone. It will work fine with an ATT sim but when i put my tmobile sim in, i can get service and full bars, make a call or 2, then nothing. I will show service but cannot make or receive calls, but again ATT sim back in, and it will work fine.

I have restored multiple times, jailbroke multiple times and nothing? What can be the issue?

Thanks

[BoisePunk22]

By chance are you in T-Mobile coverage? Just might have week signal where you live.