Vodacom cancellation

I am due for cancellation of my data contract, I was told in October to call in November to have my services cancelled. As per terms and conditions I need to provide 20 working days notice Here is my problem my contract ends on the 2nd of December so I phoned today the 2nd o November to which Vodacom is now telling me that I will still pay until the end of December. How is this ethical if I am providing my notice period, so I want my services canclled on the maturity date which is the 2nd why should I still have to pay for December