same thing happened to me, upgrade to iPhone 11 9/20, sent back my other phone the next day and had shipping confirmation through Newgistics it was received 9/30. About mid-october, I get an e-mail from ATT saying they never received it and would bill be $550 if they didn't get it back.l
So I online chat, give the tracking info, they say they got it. and would put a note in to have the charge not hit my account. Two weeks later, get another e-mail saying same thing. Online chat again, once again I am told it won't hit my account.
Get my bill a few weeks ago, it was $550 more. Chat online, first person says they got it back and would put in to reverse the charge and in a few days it would come off. Told them I didn't believe them and why can't they just put it back on. Supervisor comes on to chat and says oh no, we never received, you'll be billed and that I need to call another department.
So I call, this lady says it shows my ticket was denied as part of a promotion. I tell her this has nothing to do with a promotion, I upgraded and sent my old phone back. Puts me on hold, transfers me, then get disconnected.
Call back, having to explain the whole thing AGAIN, this guy was more sympathetic and said he'd talk to a supervisor and call me back in two hours. Called me back about 9 hours later. In that time, I called again and got someone different and basically begged him to help me. He looked at my notes and tracking, said he agreed they were the ones who lost it, went to talk to his supervisor, came back and said he had to put in the $550 credit and it would go to his supervisor to approve and that he would approve it since he had already spoken to him about it. 3 days later, I got a text saying my case was resolved and my $550 charge dropped off.
Keep trying! This was the same reason I left Verizon, I'm running out of carriers at this point. I think I may just need to buy my phones outright, but it's hard as I do like to upgrade each year.