Changing of month to month contract to 24months without authority

I have month to month data deal with cell c but They are refusing me to cancel it saying they converted it to 24months without my authorization. How do they even change contract in my absence???? I am fuming angry who must pay 3k cancelation when I had month to month and they changed it without my authority and approval, who changes contract without the person who pays. As account payer is within my rights to agree and disagree to any changes made in my contract!!!

I will not pay the 3k and I want that stupid contract cancelled