Hi yes Nokia 5310 xpressmusic is a nice phone i am using it for pat one year ..And sure Nokia 5310 xpressmusic phone is compatible with AT&T sim only if the phone is unlocked one .Else if the phone is locked to some other network provider then you can't use any other GSM sim in it until you unlock it .You can easily unlock your phone using sites like Unlocking4U.com they are rendering unlock code for all mobile models .After unlocking it freely use it with any GSM sim..