Debt-in harrasment

Constantly harassed by Debt-in for an error on Telkom's side, I have since received a letter from Telkom SA SOC LTD stating that the issue was resolved, that the account was paid up in full and all debt cleared and that the account is not indebted to Telkom SA SOC LTD. Debt-in say they are phoning on behalf of Telkom, yet refuse to speak with Telkom, they rudely tell me it is my obligation to prove it and that they do not deal with Telkom...