Harrassment from Nu Debt Management

I missed one payment with Telkom and now I owe NuDebt Management over R3000. I only deal with Telkom and will only pay Telkom the money I owe. I gave no permission to be sent to any collection company. Now I'm hounded by Nu Debt every third day. I have sent many emails with my reference number and they still don't get back to you. I'm so over this.