MTN trying to **** me!!

I have a home wifi package which I have been paying R399 for every month. I have received notification stating that there will be an increase and the average increase is stated to be between 5.4%- 9.7%

imagine my suprise when they tried to debit R470 from my account. I had put enough money to cover up to a 10% increase but was very suprised when they tried devoting such a high amount.

I called the billing department but all they do is refer me to the website. I wanted them to explain how we got to R470 but they were unable to.