It was few years ago, AT&T unlocked my iphone bought from craigslist . All they require is you still have service with them, completed 2 years contract, the phone is not stolen. When I called, I was using the iphone that i bought off craigslist, one week still remaining in the contract (my service contract, not the iphone person's contract). I told the CSR that I was going to Nepal for vacation and need to unlock the phone to use it over there. She said, since I only have one week left in the contract, it was not a good idea to ask me call back in a week, so she accepted my unlock request. I received AT&T email next day that my iphone was unlocked.

I don't know how AT&T works these days, but definitely give it a shot. Call them. If one CSR cannot help, call back again! It also depends upon person to person. AT&T has empowered its employees, so they have pretty much right to do anything. I remember calling them to waive my overage charges all the time and they always did.