Increasing of subscriptions without consent

So you sign up for a 24 or 36 month contract... Since when in the history of mobile phone contracts are these service providers just able to increase the subscription mid contract??? Surely if I sign for a R500.00 monthly subscription, Vodacom should honor that for the duration of my contract with them, instead, they increase it without any advisory or consent... Imagine I just decided to decrease my payment every month, I would be without connectivity very soon... Vodacom I want someone to contact me and rectify this billing atrocity, It is utter BS!