MTN charging wrong amounts

I had to do debit reversal this month because MTN charged an extra R1000 on top of my account. I have phoned the call centre twice with no resolve. Not even their own call centre agents can find a reason for the extra amount. I went to the store to see if I can find better clarity. They as well said that it must have been a mistake. I have been with MTN for more than three years now, only paying my contract amount. I did upgrade the previous month where I paid all fees of R2500 plus minus. Now I am receiving sms’s that they will switch off my connection. As a note. They advised me to reverse the amount. Then I paid my correct contract amount at the store.