Constant random price increases on "fixed rate" package

My "fixed rate" contract that I signed with Vodacom was secretly increased twice within one year, apart from all the increases the previous year. Vodacom did not even inform me that my rates have increased. My "fixed rate" contract has increased with more than 50% during the 2 year duration of my contract. The reason why I took out a fixed rate contract is in order to budget my mobile expenses. Vodacom has increased my account so much that it is no longer within my budget nor affordable for me. Please do not be fooled by the "fixed rate" packages of Vodacom, as these are constantly increased shortly after signing the contract.