Overcharges

Cell c has just called me to say that I have an outstanding acc with them. I cancelled my contract in feb and after a previous hello peter complaint was told by cell c that I was on prepaid platform and that the last month invoice, being the cancellation period would be free of charge. I am now being billed for this and to top it all when disputing this with the call centre agent who called me to collect the outstanding fee and after I confirmed that the call was being recorded she hung up on me. What sort of customer service is this? Is this kind of behaviour acceptable? I refuse to pay an invoice when I was told that beyond the final months charge there would be no further billing as a courtesy from cell c and then they go ahead and charge me anyway.