The DNC list does not automatically block calls. Instead, it provides a list of "do not call" phone numbers to legitimate telemarketers who pay a fee for the list.
The DNC list exempts political callers, survey organizations, charities and companies with preexisting business relationships, but these organizations might have their own, internal Do Not Call list (ask them).
The DNC list is not effective against criminals, spammers and scammers, because those people do not subscribe to the DNC list, and do not care if you are on or off that list.
Its purpose is to stop calls from legitimate telemarketers, and it does a pretty good job at that.
Here is your reading assignment:
http://www.ftc.gov/news-events/media-resource ... try/enforcement