CONTRACT IS OVER

My Mtn contract expired on the 8 January and I have been calling from last year October to check what I should do when the contract is over since I wanted to convert it to pay as you go and I was informed that I have to call back on the 8th January so as to do the changes. When I did call I was told that I will loose my airtime if I change to pay as go which I then asked why because I have paid for the airtime. So I then made sure to finish the airtime and when I called on a Saturday I was told the cancellation department does not operate on weekends and so as a result they then gave me airtime on the 2nd February 2015. I have just received a statement for Mtn and they are charging me the same contract amount of R489 (which is for the phone plus airtime) whereas I was under the impression that i would only pay R350 for the airtime since the contract is over. I have called a million times and its always different stories if not incorrect invoices with other peoples details. Please help as I feel like i am being overcharged.