How can I remove a collection im not at fault

I want to express how stressful I am trying to remove this collection from my account. I have been going back and forth with ATT/DIRECTV for 2 years and it seems like I'm getting nowhere.

I followed all given instructions from AT&T/DIRECTV on returning the equipment when I canceled my services with them in July of 2020. I even received a self-addressed postage and box from AT&T/DIRECTV  to return the equipment device which I did. I have copy of my receipt from Fed Ex showing I shipped the equipment August 22, 2020, as I was instructed by AT&T/DIRECTV.  I assumed my account was closed since I had followed all their instructions. Then 9 months! later I got a call from AT&T/DIRECTV stating that they didn’t receive the equipment I gave them all the information from the FedEx receipt, and they said that they would cancel the balance due as this was their mistake. it's not a lot of money in dispute, but why am I going to pay for something that I know I returned and have proof.   I also went to FedEx to see if they could track the package and they told me that they only keep records back to 6 months. Which made tracking of the package difficult because the time frame had already expired. The person at FedEx did warn me and said that they have a lot of people come by with the same problem. 14 months later I received a collection call for the same issue thinking again that my account had been fixed.  I Called AT&T/DIRECTV again it was September of 2021, I explain the situation and again was told they would stop the collection and zero the balance due, as this was their mistake. As you can see, I have followed all instructions as I was given by AT&T/DIRECTV  if the package got lost in transition or there was bad administration when it was delivered to AT&T/DIRECTV  that is out of my hands. I want to point out that every time I call AT&T/DIRECTV it has been a waste of time they promise they will open a ticket and have it cleared out and to follow up in a couple of weeks and when I do they tell me oh! the person that you spoke with last time started the process, but didn't finish it. So I asked to speak to someone higher than them and they can't never put me through. To this day I'm still dealing with this I'm still following up on my case to see if someone in customer service actually finished entering my case in their system. I have ran out of options and patience. I will never consider this company for any kind of service I've have had very bad experience.