HI locked sidekick mobile is nothing but the sidekick mobile is locked to t-mobile due to this it wont accept any other GSM sim until you unlock it ...Whereas unlocked sidekick mobile is nothing but the phone can be freely used with any GSM sim ...If you want to unlock your sidekick mobile from t-mobile network provider then visit sites 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 ....