Ya sure you can buy sidekick mobile and freely activate it with AT&T sim only if it is unlocked phone else if the sidekick mobile is locked to some other network provider then it wont accept AT&T sim until you unlock it .You can easily unlock your phone 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 ...