is there a way to make a cingular (att) phone work with tmobile?

i have tmobile but they dont really have the phones that i want is there a way you can switch the services?? help??
and how do i find my code

Hi you can't use t-mobile sim in the phones which you brought from AT&T until you unlock it from AT&T .You can easily unlock your phone using code .You can purchase code from sites like ClassicUnlocking.com they are rendering unlock code for all mobile models at reliable cost .You first verify that whether the code is available for your mobile model ,country and network provider .If yes then pay for it and unlock your phone .After unlocking your phone you can freely use it with any GSM sim ....