Your price increase - not the amount I signed for!

I received an email yesterday simply informing me that Vodacom will increase their prices from May 1 and that I will pay a new monthly amount from that date onwards.<br> <br> Please take note that I signed a contract for a certain amount - and THAT is the amount I will pay. I will not pay the increased amount.<br> <br> If you dare to increase my debit order, I will take this matter further.