How to unlock an Iphone

You can put in your own unlock request as a non customer but if the seller is blocking you, that probably means it does not qualify to be unlocked. Otherwise, return it if possible. https://www.att.com/deviceunlock/?#/

Good morning, I need to contact a representative of that company or someone who can help me, since they have WRONGLY BLOCKED FOR A COMPLAINT OF THEFT, that is, they have passed it to NEGATIVE BAND to my iPhone 7 Plus 128g phone made purchase on 9/6/2017 at the Ishop Mixup Quinta Alegría shopping mall 5a. Avenida Playa del Carmen MEXICO. TO MORE ABUNDATION I WILL SEND THE INFORMATION NECESSARY TO the eMAIL that you have, so that you corroborate that I am the legitimate and sole user of the same, buyer of GOOD FAITH, in an official IPHONE location, that there was never any inconvenience or complaint about it. My IMEI number has been reported for theft or loss by your company, surely there was some loading error, but since 9/8/2019 it is in NEGATIVE BLOCK BLOCKED UNFAIRLY. THEREFORE, I REQUEST THAT TO THE BREVITY, AND THAT THIS GENERATES TO ME A VERY BIG ECONOMIC AND PERSONAL DAMAGE, I UNLOCK MY TERMINAL OF THE INTERNATIONAL BASE PROPERLY SUCH. I AM WAITING FOR A SOON AND FAVORABLE ANSWER AND GREETING GREATLY. I NEED AND ASK FOR YOUR HELP AND SOLUTION. THANK YOU EVERYTHING HEART WILL BE

Proving you bought the phone only proves you bought it, not that it wasn't stolen. If the phone has been reported stolen, then only the person who reported it can remove it from the blacklist. AT&T can't and won't help you take it off. Get a refund.

You have used some terms that make no sense/incorrect.  

Negative band or negative blocked, are not terms 

    Your phone has been blacklisted.   I gather you are the original purchaser.  

To remove your phone from the blacklist if it was added in error, you need proof of purchase.  The store where you bought it should have that information.

The only suggestion I can think of is file an FCC complaint online. It’s free and someone will contact you.

If everything you say is correct then you need to contact AT&T so they can investigate it. Call International Customer Service at +1.314.925.6925 

THANK YOU THANK YOU VERY MUCH!!! THE INFORMATION YOU GIVEN TO ME IS REALLY HELPFUL!
THANK YOU