Unhappy customer

I cancelled my landline and Fibre in April 2019. Gave them a months notice, had ensure all was paid up. In May our new service provider took over our fibre connection. Telkom didn't cancel correctly on their side, I now wanted to apply for a credit card but couldn't as they have an amount owing on their side which is affecting my credit rating. Phoned numerous times, have many case numbers and still nothing is resolved.