Cancelling the Ended Contract

I acquired a 24 x months telephonic contract with Vodacom. They were debiting their monthly fee until after 24 months when the contract ended. They then sent me a bill of + - R2000 saying I am owing them and when i asked questions i am told that I was supposed to cancel the contract after 24 months which I wasn't aware. Then why did they stop debiting their monthly fee after 24 months. I asked them to send me the copy of that recordings and still nothing.