Pathetic Service

Pathetic service from MTN. I took a contract in August 2018 for R318 pm. Money was debited from my account monthly. So they started the habit of debuting the account after 2 months I disputed that because that was never agreed on the agent said it will b corrected. My contract is expiring in August 2020.now I have been told that I owe 16200 my account has been handed over. How possible is that?