MTN refusing to cancel contract without cost despite all the inconvenience.

I have been with MTN for decades as both a prepaid and a contract customer. I recently just changed my banking details. I ensured that I provide MTN with the new banking details so they can debit my account accordingly. For months now they keep suspending my account month after month. Each time I have gone back to them to request that they debit my account. It just never happens. Now I requested that they cancel my contract with no cost to me as this has inconvenienced me for months. They are refusing to do so. I have even asked them to go back to all the voice recordings for the evidence of my requests. They just recently sent me a cancellation bill of R5000 which I am refusing to pay. No one is even attempting to be helpful. It's a seriously horrible experience with this company. I guess this happens when companies are too big to care about their customers. I will be taking legal action to void this contract without cost on my end as I have not breached the contract but MTN has.