Using AT&T sim on blacklisted T-mobile GS III

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http://forum.xda-developers.com – Hey guys, I got scammed on craigslist and the guy won't respond. So I bought a T-mobile Galaxy S3 and it worked fine for over a week until 2 days ago when it stopped connecting or registering to the T-mobile network. I found out from T-mobile it was blacklisted as stolen/lost that day. I called to confirm the phone wasn't stolen or lost when I bought it. The T-mobile store says there is nothing they can do except offer me a new phone from the store to pay full price. Can I unlock a phone marked as stolen/lost using an unlock code and use it on AT&T? I tried searching the forum and (HowTos)