Unprofessional consultants

I have been receiving calls from this company telling me that i owe SARS an outstanding amount that I don’t even know about.Every time they call I explain to them that i an arrangement was made with me to pay on a monthly basis,for a period of 3months,a said amount,which i did.From November i was paying that amount until end of January.Now they call me and say i have interest,which that was never explained to me initially when the arrangement was made with.I was told to pay that same amount for 3months and nothing about interest was mentioned whatsoever.I feel very ****ed off about this.I send proof of payments already and I don’t know why they say i owe SARS.Their consultants seems to have no answers when I ask them about where the outstanding balance is coming from.All they say is “it is the interest “. I need issue solved really,