Charges after line has been canceled

I have canceled my telkom account last month (Dec 2015) and still received a bill for Jan 2016. I contacted the help centre and the lady i talked to was o rude and told me that if you cancel the line it takes 30 days to cancel but they still charge you for that 30 days. So tell me why do i get charged if telkom are slow on their side? Also the lady i talked to droped the line in my ear while i was talking to her. I will not pay the amount for Jan 2016 as none of this was mentioned to me when i activated the line. Please sort this out Telkom or i will go to helo peter.