can you use a tmobile sidekick with an at&t plan?

Hi whether you want to use AT&T sim in the sidekick mobile then you must first unlock your sidekick mobile from t-mobile network provider only then you can freely use it with AT&T or any other GSM sim ....You can easily unlock your sidekick mobile through online providers like TheUnlockArena.com here they are rendering network unlock code for all mobile models at reliable cost .You first check that whether the code is available for your mobile model ,country and network provider .If yes then pay for it and unlock your phone then freely use it with any GSM sim ...