Well, ideally, outbound telemarketers have to abide by certain regulations, like the Do Not Call list, and the reputable companies do. Scammers or fly-by-night operations, however, don't. They even spoof the caller ID, which means they use a fake number, or the number shows up as 000-000-0000, which is also illegal.
Using the Do Not Call list keeps actual companies from calling, but there's not much you can do about the scammers. They use programs that just dial numbers in sequence until they get a hit.
The best way to deal with these kinds of scammers in the US is to report them to the Federal Communications Commission. They have a complaint form set up for people who are getting these calls. I've found it much more effective than reporting numbers to the Do Not Call site. The FCC shuts these people down.