Jet demanding Higher installments on Small amount

Good day,<br> <br> I had an issue with Jet back in February where i was owing around R1900.00 and when i went to ask Cashier how much my account as i want to pay, She told me its R600.00 due. This alarmed me as i always paid R500.00 even when my amount due was zero or less, in order to reduce my amounts. I paid this R600.00 and called customer care to explain to me, the Lady of which name i have forgotten told me that i actually owed R85 and that cashier was wrong to say R600. She advised that i should check statement closely, if it says 0.00 balance i must not pay that month as i have paid a lot previously. Now statement for March had 0.00 so i decided not to pay. Now 15 March 2015 i receive statement for April, I owe jet R2257.11 and installment due on 1 April is R766.44. I called customer care ( Nombali Madala) who failed to explain why so much money, she talks about me buying R600.00 airtime which is a lie and not even on statement. My point is WHY AM I BEING CHARGED SO MUCH FOR LESS MONEY OWING AND WHY ARE YOUR EMPLOYEES GIVING DIFFERENT EXPLANATIONS. PLEASE ATTEND TO THIS AS IT LEADS ME TO CLOSING THIS ACCOUNT.