Call AT&T customer service. Be prepared with the phone number, account holder's name and the last four digits of the account holder's social security number (or have some other way of verifying that you are the rightful owner).
Once the information has been confirmed, they will list the phone as being unlockable in Apple's database. Though reports say it can take as long at 96 hours, most appear to be done within a few minutes.
Connect the phone to the computer and run iTunes. Verify that you can get to the Internet, so iTunes can get the unlock file from Apple.
Backup the phone
Restore the phone
Barring complications, the process should take just a few minutes.
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