MTN made a mistake with contract cancellation, and I have to pay R1000

Been a customer for 20 years with MTN. Cancelled my data contract and gave a month notice. They made a mistake and only cancelled a part of the contract. I kept on getting data and ,continued to be debited. January I Followed up and again in Feb and now I'm told I owe R1000. I have spoken to so many people and no one phones me back to resolve their mess. Reference number 1 - ***