Disgusted with cell c

I had an contract with cell c in 2013 which went on for two years, like most contracts do, came renewal stage and obviously I renewed to a better phone, what puzzles me is the SMS's and calls from a company called NDS claiming to be collecting on behalf of cell c from my 2013 contract , how's this possible anyway? I'm really disgusted with the service. To top it all , cell c has listed my name on the credit bureau with this outstanding balance and this is affecting my credit profile . Please investigate this as this is appalling