Stops telemarketers, takes two minutes to sign up. It is a great government service, trustworthy/reputable. If used in conjunction with anonymous call block/blacklists you can really stop all those annoying sales calls, though on its own it makes a really noticeable difference.
The US national Do Not Call Registry. I used this on my last home phone and it worked great, just know that every few months you may have to do it again. It stops all the calls, but somehow they pick up again in a few months, not a clue how or why, I just know if you do it again it fixes it!