Also, these are scammers. They're using a combination of tactics: robodialing, number hiding, etc.
The robodial simply tries every single number in an exchange. xxx-1234, xxx-1235, xxx-1236. When someone answers, they (usually) launch into their spiel.
The Do Not Call list helps with a lot of things, but often not these. They know it is against the FCC's rules, and they do not care. They will continue until caught.
Changing phone numbers doesn't even help in this instance, because they are not calling 'you', but rather every phone number.
Unless of course this is a debt collector, calling someone that used to have your phone number. That can be just as annoying.