MTN is billing clients more than agreed - theft!

I recently took out a new contract with MTN. It was supposed to cost me just under R1000 a month. It now costs me R1450. For the past two months I have been billed the incorrect amount, which was not agreed. PLUS, they have not even refunded me for the first month. I have phoned and and phoned and each time they tell me they will take it up with the billing department. Recently, the call centre agent said, "sorry mam, it's because we are working from home". Unacceptable. This actually feels like **************** - to bill someone an amount that was not agreed upon and to make it impossible to sort out.