Missing Newgistics package

I am being billed $706 for failing to return a defective device. This happened months ago. I replaced my defective Samsung Galaxy S6 Edge and had sent it in the mail and followed the instructions to the letter to return the device back to AT&T. I could not get a receipt for dropping it off because the USPS post office said they didnt give them out but i didnt worry because  i had a Newgistics tracking number and thought that'd be fine and i was dead wrong.

Weeks later the package is no where to be found. Newgistics tracking number keeps saying there is no tracking number for the values and i am being charged $706 and my service has been suspended. I had chatted with an AT&T CSR online and she had told me that they recieved the device and she will file a case so that i will be credited $706 to my account but nothing has happened. i contact AT&T again and they tell me they don't have the phone when the last person i spoke to said they did. i have tried almost everything but it seems that i will be forced to pay for a defective phone that i DO NOT have.

Do i have a case here? Is there any where i can report AT&T for this because i KNOW im not the only one with this problem. What are my options??