Contract Dispute

I have cancelled my contract with MTN at the end of March 2020 and my number was converted to prepaid by that date. They continued to debit my account during April and May then I reversed the money. Two weeks ago I went to their branch to try and make a new application and I was told that they(MTN) have blacklisted me for an amount less that R300 whereas I've been paying a phone of more than R600 over two years. Now I'm unable to get a new phone due to that.