T-Mobile Overcharged for 2 years for cancelled devices on Auto Pay and Refuses to refund

I had T-Mobile for three years. On my one year anniversary I cancelled six of the seven phone lines I had on the account because I wasnt happy with their billing and customer service. I've always had my account on auto-pay and the representative acknowledged my accounts were cancelled and I had one account I kept active for a specific reason. 

After a few years went by, I noticed on my credit card statement, which I unfortunately rarely look at myself, that T-Mobile had a charge for almost $300. This seemed like a lot for a single line so I called customer service and they explained I still had three active lines. I asked what the numbers were and two of the three lines were active on Verizon. After some research they acknowledged the lines were cancelled several months ago and I should not have been charged. The rep told me that because the credit was so large, they would need to get a supervisor to authorize it and I should see a credit on my account the next month. The next month I saw another large charge on my credit card. I called again and they had to re-review the account and told me eventually that this needed to be escalated due to the size of the credit and I should see a credit on the next statement and not to pay my bill.

Next statement rolls around and still no change. I then asked for a manager and got a foreign representative with the worst customer service I've ever experienced. She said she "wont" credit me for more than two months back and its my fault that I didnt dispute the charges earlier, and it wasnt T-Mobiles responsibility to notify me that I was being overcharged. 

I cant even fathom how to understand their logic or perspective. Its my fault I was being over charged, but when I brought it to their attention and they aknowledged the over charge, then its not their fault because I didnt find it sooner? These guys are unreal.

I of course cancelled my last phone line and will never be back. These guys dont know how to treat customers, nor do their offshore customer service people understand how customer service should be handled. I'm telling everyone I can about my experience in hopes that I deter as many people as I can away from T-Mobile and to other carriers. They've created an activist enemy for life when all they had to do was what was right and likely would have earned my business back. Go to Hell T-Mobile and your off shore customer service people!