Breaching of Vodacom contract?

Good afternoon, I've received messages from Vodacom, my cellphone contract supplier, with a statement saying that as from the 1st May 2015 my bill will be increased (on all my contracts). Surely this is ******* as I signed a 24 month period contract with them for a specific amount each month. No where in my contract is being stipulated that Vodacom can change the subscription fees. Accordingly to me...this is breaching of contract from Vodacom's side. Please correct me if I'm wrong.