want to unlock iPhone

http://www.att.com/esupport/article.jsp?sid=KB414532&cv=820

Unless you can furnish AT&T a receipt showing you paid full price for the phone, from an authorized source in the US, or that you are a current/former customer, that used that phone on AT&T's network, AT&T will not process your unlock request.

I suspect there is no way for you to get that phone officially unlocked.