Increase in contract prices

I have taken out a contract at Cell C about 6 months ago at R499 per month. I was promised by the sales staff as long as I don't go over the usage that I get with the package that the R499 will stay the same for the duration of the 24 months. In February R519 was deducted from my account and I also never received a bill as per usual for the month of February. After wasting hours of my time at Cell C's Customer Care Centre (and they also could not present a bill) I was told it is a price increase on the contract itself. Apparently you can just change a signed contract to any amount that you want according to a specific clause in the contract. I see this as highly ********* and I don't think anyone would ever do business with Cell C if they do such things. I was told by your sales representative that the price will be the same for 24 months and this was a lie from Cell C. This is also against the Consumer Protection Act which I will lay a complaint by if you don't change my monthly deduction back to R499 or not cancel my contract without any fees or penalties. If I knew from the beginning that I was to pay R519 per month I would have never signed the contract.