cellc knows how to take your money

My contract with cellc was ending on 16 of September but I called them to cancel on the 2 of September so 30 days from that date is 2 of October but why do they take my money like they say in December for November when my contract ended in October the 2nd how is that possible when I ported my number to telekom in November why did they take my money when I was not with them