OVERCHARGED!!!

The home internet contract i have is for R399 month to month, why am I being charged almost R500 monthly? And when I call the call center they're telling me about an increase communicated in December when I joined 2 months ago and prices online said it's R399 every month? I need this corrected and the extra charges added as credit on my next payment and the charges going forward to be as per contractual agreement!