MTN's Integrity related to Wrongful Billing

Firstly, I want to say, I have been a loyal customer of MTN for about 20 years and I’m at the point of cancelling both my contracts due to frustration and lack of confidence in their billing system. My complaint is simple, I upgrade my contracted, which I have had for years on 02/06/2016, but opted to not take a new mobile phone, the contract value is R719, which it clearly states on the renewal documentation and no phone was taken. I have had my MTN store request the paperwork to confirm the above. Here is the problem, from that time, I have been billed R1178.99, an additional R459.99 per month. In short, 20 months later that is R9199.8 and growing. That is a lot of money to anyone. I constantly get calls to upgrade my contract, do you really think I want to upgrade now? I have raised this issue with the MTN store I deal with, MTN Simmonds Street on a number of occasions, the last being 2 weeks ago, and even they are failing to fix the problem. I have also raised this issue a number of times with the call center and billing departments without any success, no one ever gets back to you. I’m not the greatest in keeping tracking of reference numbers, Remedy tickets etc, that shouldn’t be the point, everything should be record on my ID no or mobile no. The bottom line is, I have proof of what was signed, and it has been confirmed by MTN Simmonds Street, and my billing records show R1178.99. I want this issue resolved or I will escalate further, I don’t want the money owed to me to be passed as credit, as I don’t see myself a loyal MTN customer for much longer, I was the money back. And I want compensation for my loss of fund for more than 18 months, I’m sure MTN has gained interest or benefit not only from me, but other customers that have billing issues. If MTN is happy to take customers money, clearly from an error on their part, I question the integrity of MTN and its Shareholder promise to its customers.