Vodacom contract amounts increased!!!!

Vodacom increased my contracts with R100 According to Vodacom they can increase any contract, so why must you sign a contract? They had never told me that they can do it. So it is a unilateral change of any Vodacom contract. I had also loaded my telephone with R80 for international calls. There were still SMS'e, minutes and Data on my telephone. The next day there was 35 cent of from the R80. Nobody at Vodacom could explain it or could give me a reason for that. It seems as if Vodacom is looking for any way to take your money