[imnuts]

The phone comes SIM locked from VZW, meaning it will only accept VZW branded SIM cards. When it's SIM unlocked, it will accept any SIM card. If the phone is already unlocked, you just have to switch it from Global mode to GSM only as it will prioritize CDMA over GSM in global mode. If you put your AT&T SIM card in the phone, switch it to GSM only mode, it should work for phone calls. You'd have to setup data and MMS stuff separately though to get them to work. If it doesn't work with your SIM in and in GSM only mode, then it isn't SIM unlocked yet and you'll have to unlock it before it will work.