How To Unlock An iPhone For Other Carriers

As expected, you can now unlock an iPhone that is out of contract with AT&T. The process is straight forward and simple assuming you meet the basic requirements.
Requirements:

iPhone not in contract with AT&T, either having completed a contract or purchased without contract
AT&T account in good standing
iPhone IMEI number
If you meet those requirements, proceed:
Unlock an iPhone Out of Contract with AT&T
Locate the iPhone IMEI number and make a note of it:
Tap Settings then tap General
Tap “About” and scroll down to find “IMEI”
Call AT&T either by dialing 611 from the iPhone, or call 1-800-331-0500 and then dial 0010 to immediately speak to a representative and skip the hold time (International users dial 1-800-335-4685)
Request your iPhone to be unlocked, provide the IMEI number of the device, and then wait for the instructions to be emailed to you
AT&T will initiate a request to unlock the iPhone and then you have to restore the iPhone through iTunes to complete the device unlock. With the device unlocked, a micro-SIM from T-Mobile or other compatible networks will now work in the iPhone.