We can add money without informing you

Yesterday I receive mails that my contracts with vodacom will be increased. I phone vodacom to confirm that this is not a hoax and they inform me that my contract with them stipulates that they have the right to increase the contract value during the period of the contract. I then go through my accounts and pick up that my Top 99 which I was paying R78.61 (as per 24month contract) was moved up to the amount of R108.50 in November 2015 without any notice to me. The said contract was up for renewal in October but I decided to leave it on the existing contract, Vodcom advised me that I a welcome to not take an upgrade and request only a data fee which will lower my fee. After must frustration I request to speak to Supervisor, Theo the supervisor failed to inform me why the contract was upped from R78.61 to R108.50 without any notice to me, I then insisted to speak to his manager, he refused bluntly to put me through and agreed with me lodging a complaint on Helo Peter as Vodacom has the right to update cost as they please without informing customers. I kept on requesting to speak to a manager he kept on refusing and after a fight he said his manager will phone me today. No call!