Month to month contract turned into fixed term contract without my approval

I had a month to month data contract which I ok last year with cell c, I tried to close it this week and I was told I must pay in R4600 to cancel as it is now a 24 month contract. The lady told me from January this year all month to month contracts were converted to fixed contract. I did not agree to this and I do not want a fixed contract hence I took the month to month in the first place. Iam not happy and do not want this contract they are giving me.