owning up to your mistakes

September 2022 I was giving a promotion a deal on over the phone with a customer service rep as well as the supervisor. However, I was told that because a promotional deal the iPhone SE third generation was going to be added to my canon. Oh cost later find out that I'm being billed for the iPhone SE third generation of my account I called roughly about twice a month for the past year, and even though, I requested that they review the phone call when the promotional deals give it to me. They tend to ignore me. I've been promise by multiple supervisors and even manager and they will call me back and manage my account properly and they will follow up with me a year later now August 25th 2023 I'm still dealing with the matter. Even though I told my story multiple times to customer service rep, managers, supervisors, etc. they just pause and somehow turn it back on me. The right thing to do here is clearly owning up to the mistake that they did. It's not a problem my end no I have a phone that's out of service and they're wanting 400+ dollars just to get my service current again. It's a shame that even though I was like to still demand payment what can I do?