MTN overcharging me for my contract by close to double the contract amount

I have been having a contract with MTN for more than a decade now , 5years back I upgraded and added an insurance to the phone , 3 years back when I did another upgrade I cancelled the insurance as I did not have the mobile phone anymore and was upgrading to another mobile.

Last year 2020 July MTN started overcharging my billing amount all of a sudden by around R200, I paid no attention to it and continued paying , then they starting overcharging me by more money I still paid and didn’t ask, my insurance was not more than R150 they one I cancelled 3 years back.

December 2020 they overcharged me by close to R600 and started suspending my service, when I went to query this , I was told it was for the insurance and VAT, I have not been insured with them for 3 years and now they decided to add that insurance money on they own.

MTN has been overcharging me and they even threaten me with SMS’s saying my service will be suspended if I don’t pay the outstanding balance or the overcharged amount.

MTN is making me pay for their incompetency , I want my account to be credited with the money they have been overcharging me.

Regards SC Thabe