Bad Service from MTN

Truly disappointed with MTN. I have a contract phone with them . I paid R300 per month. I then went to Heidelberg Mall MTN to pay the amount of money that was left for me to finish my contract. I then requested to cancel the contract earlier as I had paid for the phone. I was then told to call cancellation department, which they picked up after a few hours. I then told them that I wanted to cancel my contract and they told me that it was calculated incorrectly and that I had to pay another sum of money. I am highly disappointed with MTN .