Telkom unjustly handing account to Nu Debt

I have been a Telkom customer for many years now. All my payments have been made via debit order. I have cancelled one of my lines and was given a confirmation letter for this in February. The telecommunications company has been double debiting my account and then they hand my over to Nu Debt who have been harassing me with calls and emails. I have contacted the Telkom account department to look into this matter and provided my bank statements, cancellation letters to prove my case. All the accounts personnel I have spoken to do agree that my account is up to date and that my account should have never been handed over to creditors. They all promised to escalate and get it sorted, but I'm still being harassed by the creditors with calls and threatening emails. I went into a Telkom outlet to speak to a service agent in person. He kindly went through my account, and he agreed that my account is in good order. I am beyond disheartened by the level service and treatment and can't start to imagine how this is negatively affecting my credit score for good reason.