unlock

Hi, I bought a cell phone that had service with AT & T thinking that it would work with AT & T Mexico. and I already realized that I have to unlock it. When I enter the page I get the error: it seems that the device you want to unlock is active in another account at & t.

I no longer have the way to contact the owner to ask him to cancel it, he give me it t with everything box manuals charger etc. but it seems to me that he did not know, nor I of this type of blockade. what I do?