Help! Customer Service agrees my records show I returned a phone, but they say they are helpless to help me due to their broken system

I am looking for help and I don't know where else to turn to at this point. Is there any way to speak to someone at AT&T when none of their normal channels can help you despite their best efforts?

I bought 2 iPhones on 6/6/2019 with a BOGO promotion. Only a few days later, I returned both unopened iPhones because the salesman gave me the wrong information when I ordered them and the net result was my one daughter who already had a phone number would have to switch to a new phone number. I ahd to return them and order two more a week later. I returned the two iPhones in one box but somehow, one of these two iPhones was never logged back in at the receiving warehouse at AT&T. I noticed right away that I had an extra phone line I was paying for. % lines instead of 4 lines for 4 people. Customer service told me to keep paying the bill and it should all straighten out eventually. But it never did. I called every month and eventually a customer service rep was able to determine that I did indeed return the phone in question, and she issued me a credit on my account at this point, for all the months so far that I was paying for the monthly iPhone payments and the phone line linked to that phone that I never used. The credit was a relief as this was AT&T acknowleging that I was paying for a phone that I returned, and I thought the case was closed. Yet month after month after that initial credit, I kept getting new bills for this same phone! Every time I called, Customer Service had to read through my very long case, and eventually they would agree that yes, I returned the phone. Seven months later, I am still being charged for this phone I returned a day or two after I bought it, unopened, never used. How can I reach someone in upper management? Managers of Customer Service or Retention never take calls directly, and they will not give phone numbers, nor do they ever call me back no matter how many times I am promised a call from a manager. Every AT&T employee who ever studies my long case history will eventually tell me, out loud, that AT&T is wrong, and that I should receive full payment credit for the phone they lost and all the payments I ever made on this phone and its phone line. They tell me the conversiation is recorded, so there are dozens of recorded calls with Customer Service appologizing to me and promising me they will do their best to get my issue resolved. One of them today told me they are very sorry because they see I returned the phone, they stated that as official representatives of AT&T, I should be creditied in full for every payment I was every charged for this returned phone, but that sadly because of the automated way that AT&T conducts a search for a phone, the company will never stop billing me! She said an automated system will look for the phone's ID# and when it doesn't find it, my case will be closed and a bill will be generated every time. EVEN THOUGH THEY ADMIT THIS IS A MISTAKE. Time and time again! I am so frustrated by this madness. It is right in my case history for any AT&T representative to read. They want to help me. Instead, because of their broken system, the customer will be screwed. This will soon go to a collection agency and this will effect my credit, and if I do not pay $600 that no AT&T representative believes I actually owe AT&T, my 4 phones lines will be shut off. The customer service rep today told me at the end that there is sadly no way for me to receive fair treatment, or talk to any person with the resources to simply override the broken bureacratic automated system. She was basically hinting that I should just pay the $600. What kind of company creates a customer service system for the public wherby there is no way to escalate a case to a higher level??

All I want is for somone to read though my case history and actually give me the credit that has been attempted so many times already. Plus maybe a few grand for the hours and hours I've spent on the phone for something that was not my error!!! As a loyal member since 2008, who pays for 4 phone lines, it is really in AT&T's best interest to not lose me over a $600 bill in their error, because they lost a phone I returned 7 months ago that is now outdated and unsellable anyway. There are some very nice people in Customer Service and the loyalty department who have spent over 20 hours on the phone with me trying to work this out. Two separate cases were opened and then closed automatically when the system could not locate the missing phone ID. I am hoping that someone at AT&T will spot this and pass it to some higher secret department or something. The next steps if this leads nowhere will be to reach out to my source at Gawker Media/Gizmodo, the tech blog, as that will certainly gain more attention if they publish my story. But I believe in playing nice first and giving people a fair shot to do the right thing.

-Frustrated Dad