Hi yes blackberry 8830 mobile will sure work with AT&T sim only if it is unlocked one .Else if the phone is locked to some other network provider then you can't freely use it with any other GSM sim until you unlock it .You can easily unlock your phone using sites like WickedUnlock.com they are providing unlock code for all mobile models at reliable cost .You first check that whether the code is available for your country and network provider .If yes then pay for it and unlock your phone....