Instalments fluctuation in spite of top up accounts

Ok im now seriously fed up with Vodacom. When i paid R4200 two months ago i was told it was because of the connection fees and upgrades and what not. I paid R3700 something last month. So how the hell can we be back to R4200 for this month when all of the lines i have are supposed to be Top Up lines. If this is not sorted out first thing tomorrow, i will put a stop order on my account or simply draw all of my money and you will not get a cent.