Terrible service

I bought a contract cell phone with Telkom in April 2015 and returned it within 5 days and I got a return document. However Telkom was still billing me in October/ November 2015. I called them several times and visited their branches and they promised to sort it out. A lady from Telkom did call and promise to help me out. But now I am getting messages from NUDEBT ref *** threatening me about Credit Bureaus and telling me that a trace locator being added to my profile. How could this be if the matter has been resolved? How am I going to apply for finance in the future with this stigma on my name? Why was I billed in the very first space if I brought the phone back? Now my name will be dirty. I want this Telkom to call all their debt collectors and MAKE SURE MY NAME IS CLEARED ASAP PLEASE. I cannot tolerate this anymore