and if they still do ring after you've told them you're on the DNCR you can get your telco to track them and pass on the details so they get reported for it (we had to do it and this one company's number wouldn't come up on our phone as they blocked their number but Telstra (our provider) was able to track down who they were and report them
or you can simply ask what company they are calling from, note down time and date and report direct to DNCR. they are really on the ball with tracking it - i have had two call through, and have had letters a couple of month later about the result of legal procedings in class action cases against the companies and the fines they've received