MTN over-Billing

I have been trying to get my mtn account billing fixed, they took more than whats on the invoice and they are just reluctant to help me fix it, is should just stop this debit order nonsense and close the contract. A service agent called Ntsako Hlongwane and another one called Pinky Ngeleka could not manage to handle my complaint. If you phone them their systems are always down/offline