I'm guessing scammer don't bother to check DNC lists, and they're not "buying" phone numbers. A computer can easily cycle through every possible phone number combination. If you answer, or even if it goes to voicemail, they now know that your number is a valid combination. They now use that number to hit you up from different calling locations. It sucks. Unless you have a white list, where you are only accepting calls from known numbers, you're going to end up on someones list eventually.