Account ******* Vodacom!!!!!...

I honestly don't understand why I have to contact vodacom endlessly and still no one can provide me with decent detailed info regarding this vodacom account....Account was converted to prepaid for 6months, R800 payment was made, then R1000, and then R2740 on 13 December when the lines was converted back to contract.....now to my shock I receive another account for the amount of R2713.........I mean this is ridiculous to say the least, will you keep milking customers who are already suffering from high costs in this country???? How do you get to R2713 for end of January?????? I know exactly how much each line is supposed to be, so how do you make your sums?? No vodacom you are ripping customers off and provide no explanation. Every time I phone the call centre they only say this is what the system shows..........Ridiculously so.......if I make my sums the monthly bill should be R1400???...where did the rest come from?