Forget the do not call register, for as you say charities are exempt. However, if it is a private number, you can always go to a silent number. I did and never get calls from charities. I always thought that they just called numbers sequentially, but apparently not.
Silent numbers do not show up on any lists. I worked for Telstra, as a corporate tester, several years ago and even their people were not able to access numbers unless you had authority to access the right system. It simply came up on the screen as a Silent Number. They were very strict as to who had access.
Having a silent number is great, as you only give the number to those whom you want to call you. When I went silent I also changed the number.
Unfortunately, it is not the same for my mobile. Two weeks ago I got an SMS saying that I had won $2 million US in a free European lottery. All I had to do was sms my name and number. Doh! They sent me an SMS asking for my number
And no, I didn't answer it. You do not get something for nothing in this life and so the old adage still applies "if it sounds too good to be true, it usually is!"