But a used phone saying it was unlocked

Hello @GorditoKev, we can shed some light on unlock eligibility requirements. 

Non AT&T Customers can unlock a device as long at it meets all eligibility requirements, including not being reported as lost, stolen, or involved with fraud and not active on another AT&T account.

Since the device is on an active installment plan you will need to return the device back to who you bought it from as we will not be able to unlock the device as @formerlyknownas, has stated. 

Thank you for reching out to us. 

Crystal, AT&T Community Specialist 

There's nothing AT&T can or will do. Just return the phone to Amazon.