Incorrect billing (***) and contract cancellation

I used to have a contract with MTN and it expired earlier this year. I was then on a month to month basis and billed R198.95. Then all of sudden starting end of June my bill skyrocketed to R639.79 and I have not received an explanation for this after countless emails (***). I even went to the store to enquire about this they also could not explain. It has been 10 days now since I enquired about and I have noticed this month's bill is also R639.79.

I would like this month to month contract cancelled and be converted to pre-paid.

Regards M Tukulula