And if you do still get them (cos some how a few always sneak through), as soon as you say "thanks but I'm on the DNCR", they hang up like a shot. They can be fined big amounts for not updating their lists and calling people on the register.
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