Trying to cancel contract

I have followed correct procedure and informed tracker that I will be cancelling my contract with them on the contracts natural expiry date at the end of January 2016. I was called by the cancellations department already and have discussed my reasons for leaving. Since then, i have had a few missed calls on a Sunday, followed by an sms saying that tracker has tried to contact me regarding my cancellation, but could not get through, and i should call them back. I have already fulfilled my obligations in providing notice of my impending cancellation, and have no need to call tracker back. I would like tracker to note that should i be billed beyond my end of contract, i will need to take further action, as i have notified them to cancel the contract. I do not take well to the fact that the cancellation sms informs me that i will be continued t be billed until after we have spoken.