How Can I Unlock My C139 From Cingular(now AT&T) So That I Can Use It On A T_Mobile Network?

Often times, if the phone is compatible with the T-Mobile (or any major carrier's network) they would do what is called a SIM unlock on the phone. It can usually be handled over the phone by speaking with Tech Support and takes between 2 and 72 hours to have handled, depending on phone model (whether it is a RAZR, Blackberry, etc.) and the original carrier.