For starters, remove your iMEI for your own security. Second, you need to call ATT to see if it is locked to ATT. Third, since you are not nor have been a customer of ATT, they are probably not going to unlock your iPhone per their guidelines. I would reach out to the seller and have them request the unlock. The phone number is below in my signature. Use Skype to keep you calling costs down.
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For service or support questions including existing order status, call:
Customer Support: 1-800-331-0500
Information on iPhone unlocking can be found here by copying this link into your browser:
http://forums.wireless.att.com/t5/Apple-Community-Discussion/iPhone-Unlock-Information/m-p/3260825#U3260825
