Put the phone in Global Mode stuck a friends AT&T SIM in it was showing AT&T network on top and was able to makre calls just fine! I think the "Global" setting just means GSM. You should also be able to use Pay as you go GSM SIMs as well.
Being a 'world phone' most the world uses GSM so yes, did you happen to check the LTE function for data, I believe there are different freq for the carriers this may not work on ATT