Do not take out a contract with MTN! They will damage your credit score after you cancelled

Never take out a contract with MTN. Cancelled my contract end Sept 2020 and now that I app**** for a loan there is a dispute against me for outstanding amounts logged 28 Jan 2021. How do I fix this now? They never answer their phones and all was paid up when I cancelled and even had to pay a month after the cancellation.