Can you use an ATT sim card with a non ATT phone?

i believe so, if the phone (unlocked) would be operable if, the GSM version aligns with the providers GSM offering. in other Words if an AT and T go phone sim card originally worked through a AT and T phone, and operated on the GSM 1900 bandwidth, then the unlocked phone would have to be capable of working on the GSM 1900 bandwidth. The phone and sim card originally come programmed to operate on a predetermined GSM code and these numbers need to match for swapping to work.