Third Party Calling for amount that is paid

Every month i get a call from Nu Debt Management to tell me that my account is in arrears and 99% of the time the account has been paid, however when they call they always tell me i owe double than what is stated on my invoice. Why is it that my account is handed over when it is not even 30 days in arrears!!! I think this is unacceptable and the customers should not be harrassed like this. I got a call the other day to say that I owed over R5 000.00 and when i called Telkom Mobile i was not even in arrears and when i told Nu Debt Management this they said they owned the account and i needed to pay this and then deal with Telkom. Why must i do that??? I want to know stopped being harrassed by this company......