Sudden increase in contract price mid contract??

Since when can Vodacom increase the price of a contract in mid contract??? There has been no warning, no negotiation and as far as I am aware the price is fixed for the period of 24 months. What if I suddenly feel like paying less? I see from your facebook page it has happened to most of your customers - with no warning or negotiation. I am going to change the debit order to a stop order and make sure you cannot increase the amount since you are def not a company with which I choose to have a debit order any more. Nowhere in my contract does it state you are entitled to raise the cost of the package at any stage. This increase is *******.