Incorrect billing

Mtn is the most useless service provider. Their billing system is up to **** and they expect us as the consumers to accept their incompetence. I have 2 contracts with them. My cellphone and a data contract. My subscription is R159 a month with 2gigs data. I hardly ever use my data, so since inception of the contract I have paid the subscription only. For March and April, they claim that I have used R14671.47 worth of data, the second month I didn't even use any data, but my bill was over R6000. They also claim that their accounts department has looked at my query and came back that the billing is correct... What utter nonsense!!! MTN needs to resolve this!!!