Ya sure the sony ericsson W990i mobile will work with AT&T sim only if the phone is unlocked one else if the phone is locked to some other network provider then it wont accept AT&T sim until you unlock it ..You can easily unlock your phone using sites 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 country and network provider .If yes then pay for it and unlock your phone then freely use it with any GSM sim ..