You might try our Android cell phone forum:
http://www.tomshardware.com/forum/forum-36-345.html .
Best thing to do is to ask ATT to unlock it. I haven't checked in years, but there was supposed to be a law requiring them to give you the code. It's going to depend on a unique identifier that you get from the phone.
Failing that, you can google for phone unlocks. Many, many years ago I unlocked a phone with a code I bought online from a company on Malta.