When I get an unsolicited call, I tell them to take me off their list. Some of them, if you listen to the recording long enough (instead of pressing 1 to tell them to take you off the list) offer a number to push to get off the list. I rarely find that works, or there are so many out there that others call in their place.

If I find I missed a call I enter the number in a site like http://800notes.com/, or without even doing that, just enter the number in the search bar of the browser. Use the hyphens so it recognizes the format as a phone number. You'll see other reports on the number and can usually figure out if it is real or not.

SRS