Ya sure you can use your t-mobile sim in the sidekick mobile only if it is unlocked one or else already locked with t-mobile network provider else if the phone is locked to some other network provider then it wont accept t-mobile sim until you unlock it .You can easily unlock the phone using online providers like TheUnlockSpot.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 mobile then freely use it with any GSM sim ..