Unfair billing & contract not according to what was signed for

I would like to cancel this contract as well. The original agreement was that I can purchase Cell-C Airtime when mine is depleted. Now all of a sudden I get a invoice of R363 in stead of my contract amount of R139p/m ???!!!! I did buy my own airtime and data when it was depleted ?! So how do you bill me for things I bought. And now when I called the Call Centre - I was told that I can't use Cell C - and that I can only buy airtime in a MrPice store??????? So if I run out of airtime at 8 o'clock at night - there is now way that I can recharge my airtime or data - and the respons was NO !!!!! How is this possible - this was not the info I received when this contract was offered to me ! I want to port this number to my old provider. I need my old number as I use this for work purposes !!!!!! Give me a new number - so that I can throw this contract number away. I would appreciate it if you can get back to me ASAP