ATT can't find my equipment I sent to them. Why do I have to pay?

I had ATT U-verse for several years off and on year and had to close my account because I moved. I returned the equipment per the instructions in April. ATT says they can not find it. I have the Tracking Number from UPS that says it was received April 6th.

As you can see in the image, I have called 10 times over the past 6 months to get this resolved. Yesterday I was informed for the 10th time that they will escalate the issue and investigate. Each call I have given the tracking number (Why can't someone write it down?) and told that it will be resolved.

This last call, the customer service reps would simply transfer me to another rep (not a higher level one, just another one) where I'd have to explain the whole situation again.  I spent an hour and a half either on hold or talking to 4 different people.

I have never raised my voice at a rep or expressed anger at them. I am at a loss as to what to do. I returned the equipment, kept the tracking number, and patiently tried to get this resolved, but in the end, I am told that if they can't find it, I still have to pay $481 for the equipment.

Someone explain to me why?