GSM/UMTS, but unlocked? I dont recommend it. You`ll be able to use voice, but they will no offer very much support for unlocked devices, and ATT does not garentee any srevices will work. Not to mention that unlocked phones from other providers typically have preconfigured setting that cant be modded to use on ATT. Stick with an ATT branded phone locked/unlocked. But avoid Black Berry phones, ATT does not support data on these phones. No iphones. In fact avoid SMART phones all together. There are plenty of 3G comp phones that are not SMART or PDA.
