IPhone locked

I talk from Brazil,so : I bought an Iphone 5 at Panama when I was in bussines, the seller told me that the IPhone was unlocked.When I came back to my country I figuered out that the IPhone was locked.

I tried many times to unlocked this phone by "ATT Custumer Care About Unlocking Device" but this department answer the same message every time that I send them a fax. BesideS I have sent them all the requests (original reciept, request number and IMEI) The message is:

Thank you for contacting AT&T Customer Care about unlocking your ATT Mobile Device. We are unable to complete your request at this time due to the following reason: Faxed documentation is ineligible or not received.

-What can I do to solve this problem?

I can't returne to Panama.

-Could you help me?