Billing incorrectly

I took a contract phone in December 2015 with Cell c which is Epic 100 at 169.00 and I was advised that the contract is an open line, then I have to phone the consultant and ask them to limit it for me. I phoned them and they promised that all that was sorted out and my February bill will be 169.00 as agreed. To my surprise when I received my statement now it says that I have to pay 574.06 which is way higher than my January bill. I don't understand if Cell c is using this open line contract to rip us off or what is it with them.