Find your IMEI Number:
Call AT&T or go to there chat support:
Contact Number: 1-800-331-0500
Chat Support: Click Here
Just ask the AT&T Representative that you need to unlock your iphone .
You must give the representative your IMEI number.
your device can’t be associated with a current and active commitment on AT&T, and you must have fulfilled your contract term upgrading under one of their upgrade policies or paid an ETF (Early Termination Fee).
The Representative should tell you everything else you need.
Once your done you should receive an unlock code and direction on how to do it
through email or phone call which ever one you wanted them to contact you by.
Tell us in the comment section if you have attempted to call AT&T and what results you have gotten from it.