Hi only prepaid cingular GSM sim can be freely used in the phone which is locked with cingular you can't use any other prepaid GSM sim in it until you unlock it from cingular ..You can easily unlock the phone through online providers like SimpleUnlocking.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 phone then freely use it with any GSM sim ...