Incorrect billing - Furious

I did an early upgrade with MTN online. I selected the option to pay the device for the remaining 24 month and now I received my statement and MTN is charging me for an early termination charge. I did not terminate my contract. I have been with MTN for way over 10 years and this is the first time I am experiencing this. No one in the call centre can assist me as it is not their department I need the correct department to sort this out as I WILL NOT PAY THIS AMOUNT !!