VODACOM *****ing money!

I have signed a vodacom contract stating that I will be paying R580 a month, for the past 5 months I have been paying R760 a month. I called the first month after which I was told the first payment is more due to extras etc, which was fine and was told thereafter I will be paying the normal R580 per month that I signed for! The next month I was charge R760 again and phoned again I was told this is a mistake and should be fixed! Ever since I have been paying R760 and the reply is always this is a mistake and we will fixed it but They never do!!!!! I will keep on writing post and bad reports on every site till I receive correspondence! <br> <br> How can this be legal?