Unlock Phone

If you don't know who the owner is, who did you purchase it from? If it's blacklisted there is nothing you can do other have the original owner remove it from the blacklist or get your money back.

Thanks for the reply
I bought it from Romania on the internet. I checked it on different sites for imei and it doesn't seem to be blacklisted

Even if not blacklisted it won’t be unlocked until the original owner removes it from their account. 

Thanks for the reply
And how can I contact him?
Can the AT&T support get in touch with him?

Now I can't even contact the person who sold it to me. And I really don't know what else to do. I can't even call AT&T support.

I doubt if AT&T Support will contact the customer. You just may have to write it off as a loss.

I have the phone, its box.
The phone is clean.
My mistake was that I didn't check the card and I ended up in such a trap. I can't pass it off easily because it cost a lot for me.

Another problem is that I cannot contact AT&T by phone. I cannot call from Romania to America

Sorry, that’s the risk of buying phones second hand. ATT will not help you contact the original owner. Have you tried putting in an unlock request? 

I understand!
How can I submit an unlock request? (Refer to https://www.att.com/deviceunlock/unlockstep1)