Device return - disconnect between AT&T and Newgistics

I traded in my phone and mailed back the old phone over a week ago and tracked it through USPS and Newgistics and have the confirmations that it was delivered back to AT&T several days ago however I continue to receive text message and email from AT&T stating my phone has not been received and I will be billed if they don’t receive it soon. This same thing happened when I traded in my phone last time and they won’t take the return at the store and despite having confirmation from the shippers that it was delivered to AT&T I am still being held responsible for the amount. I need assistance making sure my account is not billed.