Keep this is mind and follow it exactly: DO NOT TURN ON THE PHONE WITH THE AT&T SIM CARD INSIDE IT!! It's inserted by default and the moment you turn on the phone AT&T considers it as a prepaid device activated on its network and doesn't let you unlock without using it for 6 months.
How will they know the phone is on? It won't be connected to their network...
But i will take your advice and read the IMEI of the back of the device or box and not turn on the phone (without the at&t sim card :wink: ) until they *hopefully* send me the unlock code