If you get one of these calls, just hang up or let it go to your voice mail.  Put a call-blocking app on your cell phone, preferably one that allows you to flag spam calls (I use Hiya - it's free).  Do not press the "do not call" option on the message!  That just tells the robo-dialer that it has reached a live human, and the calls will ramp up even more.

Something that has dramatically reduced the number of repeat calls for me is using the stock voice mail greeting in which the generic "robo-voice" announces only my number in the outgoing message.  Most robo-dialers have voice-detection software that can tell the difference between a human and computer-generated voice, even on a voice mail greeting (and they're obviously looking for humans).  Now I notice that a robo-dialer only call once or twice, and almost never leave a message.  My theory is that the software detects the "non-human" greeting and associates it with a non-working number (or at least a number that leads nowhere profitable for the scammer).