Written off debt affecting my credit score

After 2 years of debit orders Telkom manages to mess up my last debit order and double charge me. They also failed to send me an invoice on the final whi h resulted in me reversing both debut orders as the amounts were different. A month later they add penalties and told me to pay after the ***** up they did.

Fast forward 6 months later: credit score down by 60 points, home loan requires 15%deposit and telkoms handed o er balance for collection is sitting at R1 000.00

On a ****** R150 FINAL installment that could have been avoided.