Yes, my mother in law received one of $7000 tax calls just the other day. She was suspicious and said that the man gave her the number to call back on and after 10 min he called back saying "why haven't you called", she said "because it's obvious this is a scam" he swore at her and hung up.
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.
There was a change to the law several years ago to stop sequential and random dialing. I used to have a silent number and was pestered until they made that change.