Vodacom -17% line rental increase

I have signed for a 24 month contract "Smart top up S" at a cost of R244.74 for line rental per month excl vat. I am approx 1 year into this contract. Vodacom have now increased my line rental by 16.9% to R286.09. Just like that....and no, I did not get a notification. How is such a whopping increase even justified and on an existing contract? Is it not enough that we are constantly having to battle data issues and overpriced rates and now this?