cell c’s dept collectors abusing me with calls for a paid up account!

I canceled my cell c account a couple of years ago, about a year later css started calling me. Regarding outstanding money… for a few months the kept on contacting me but could not provide me with a reason to why I owed cell c money, they advised i call cell c direct. Eventualy when i spoke to cell c they provided me with a reason and an amount, i paid it… CSS is abusing me with phoe calls daily even after I have give them the prove of payment a number of tiea times. this is abuse and eligal to ask me for money of a paid up account can cell c please sort out their dept colectors