Harrassed by RCS claiming I am owing them

I am being harassed by RCS for the past week and they claiming I am owing them R334.62 and their rude agent is keep on calling and said they keep the records but I still being harassed and what irritate me most is the fact that I requested paid up letter and was sent to me on the 28th of May but today I am being told I am owing them. What nonsense is that, owing RCS for what? And their irritating calls you answer for 20seconds no one speaks. I want this be sorted as a matter of urgency, even to get a letter I have to log a review for me to get my letter after a week payment was made this is ridiculous and unaccepted behaviour in a client servicing company, even last time I had an issue with your rude agents