charges for money that has been paid

I am mortified in the manner that RCS conducts business - in December I took out a personal loan of R24000 for my CTM account. I have since then paid 2 installments of R1009 each - and R17900 ... Which equates to R19918 in total - this was paid within 2 months of opening the account. When I contacted RCS at the end of Jan - they informed me I only had about 4 installments left. All was going well until the 23 Feb - I received an sms stating that I owe RCS R19600!!!! I phoned back a total of 5 times - and RCS claims that the figure is correct as I have to pay interest, admin fees and insurance on the initial amt app**** for - they claim that the initial debt was R24000 but the prinicipal debt is R38600 over 48 months. I keep telling them that they cannot charge me interest, admin and insurance on monies I have already paid in ... However, 6 phone calls later, they r still adiment that the amt owing is R19600!