Fleecing by MTN

I have three contracts with MTN and I have been with them for years. Strangely now they are over billing me even if I have a contract which is debited every month. They claim I'm in arrears, how can one be in arrears while there is a debit every month. I September they never debited the correct amount of R696. instead only debited R93, then October they came and debited double, I obviously had no problem with that, but my biggest concern is that it was their incompetency. now my billing November is R801 because according to them I have to pay R105 extra for their incompetency.