Lots of "illegal" and stolen phones - especially Apple phones - wind up overseas just the way you described. You wll never get an unlock code from an American carrier.
For one reason - you can't provide a receipt from an American carrier that you paid for the phone in full!!!
And you aren't a customer and never were.
The odds are good the phone is "blacklisted" and never could be used in the U.S.
As we say in the U.S. "You have a lot of chutzpah." (Guts) for even asking.