NuDebt Management is a ****?

Good Afternoon. I was contacted by this company saying I have been handed over to them from Cell C owning X amount of money. I contacted Cell C and discovered I did own them some money but not as much as they stated. I have paid Cell C now an got my paid up letter which I have now forwarded to them but yet I still get sms an mails from them Any1 ever felt with them?