Ya sure you can use the sidekick mobile with any other GSM sim only if it is unlocked one else if the phone is locked to t-mobile then it wont accept edge sim until you unlock it ...You can easily unlock your sidekick mobile through 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 the sidekick mobile then freely use it with any GSM sim ...