Using an ATT sim card in a T-Mobile prepaid phone?

No you can't use AT&T sim card in the phone which is locked with t-mobile until you unlock it .You can easily unlock your phone using sites like Unlock-Free.com here they are generating network unlock code for some older mobile models at free of cost .Else if you want to unlock some other new model phone then visit sites like Unlock-Zone.com here they are rendering network unlock code for all mobile models at reliable cost ...After unlocking the phone you can freely use it with any GSM sim ...