Data contract not cancelled

I app**** for a 24 month data contract in December 2012 at R99 per month. I was under the impression that it would cancel automatically after 24 months. It has not but cell c is now deducting over R300 per month for something I have no knowledge of. I haven't received any invoice to inform me why they are deducting that money. Kindly inform me why I'm paying for something i did not agree to. I agreed to a 24 month data contract that hasn't been cancelled. Is it my duty to cancel the contract? I'm not willing to pay over R300 for something that was paying R99 for.