+1

You mentioned you falshed the Radio so I am assuming you are on Radio 2.32.00WU ( 'U' being unlocked and not V or F ...) After witch you will need to be SIM-Unlocked. Then you will insert T-Mobiole SIM Card and manually switch 'Global' Mode to GSM ONLY and it should register onto T-Mobile.

(DO keep in mind that if you boot into Android with the SIM card in place, you will be on T- Mobile and not Sprint)