Apple 16GB iPhone 6s Unlocked GSM (S&D)

Beware!!! Some of these phones are not unlocked. I bought one a few months ago and can only use the phone with AT&T cause every other provider has the phone blocked and cannot unlock it. The previous owner of the phone did not pay his bills and the phone is forever banished by every provider he screwed over.

How much S & D did the phone have
Is there a way to check if the phone will truly be unlocked

I also want to check if it is really unlocked