Be fair and honest to your clients

Good morning I have no arrears on my account and I pay well every month but instead of the pay to buy increasing it decreases always March I paid R1300 April R1300 May R1298 total is supposed to be R3898 but to my surprise total amount paid now is R2790 what happened to the rest of the money paid? +-R1108 all your interest etc are added on the monthly installment which I pay on time every month I call your customer service line and the lady hangs up on me once I start addressing my issue