Unasked for price increase

On the 23rd May I sent a letter to Vodacom with reference to my Smart Top Up contract. I stated that I wished to register my extreme displeasure at the increase of R40.87 per month on my account. To date Vodacom has not made any effort to reply to my letter, nor have they adjusted my account. After numerous phone calls I managed to establish that Vodacom has introduced this increase to allow me to have more data? I was informed that it is in my contract that Vodacom reserve the right to increase fees should the need arise. What need??? As, this was something that was not made clear to me when I negotiated my new contract, I object to the fact that with out any personal contact this increase was never the less implemented. I believe that a circular sms was sent out; something that I did not receive as I was in Australia at the time, saying that a price increase would come into effect on the 1st May. This is totally unacceptable. I do not believe that anyone has the right to summarily increase a contracted price. I am a pensioner living on a fixed income and when negotiating my new contract I specifically asked that the negotiated price would be for the duration of my 2 year contract. I was assured it was the case. I wonder if anybody else has had this same problem.