Unlocking an iPhone with AT&T Web Tech Support
Go to AT&T’s online account tech support page, login with a valid AT&T ID, and click on “Technical Support” to start a chat with a live representative.
State that you would like to unlock an off contract iPhone and provide the iPhone IMEI number.
Let the AT&T representative check if the iPhone is indeed without a contract and give them an email address when asked
If everything checks out, the AT&T rep will provide you with a case number and an estimated date of completion for resolution. The date they provide is a rough estimate and by no means represents the date you will get an email from AT&T. Sometimes AT&T sends the unlock emails out within an hour but it can take up to 10 days as well, the time varies widely based on the volume of unlock requests. I have completed this process several times with multiple different iPhone models and it has worked every time, though the time between request and completion has varied.