Otherwise I either let it go to voice mail (which is nearly always full, so good luck leaving me a message, which I might or might not check within a month or two), or use the Google call screening service.
Land-Line: Answering machine contains just 'This line not active' set of tones plus a terse 'Leave a Message' audio. The robo-callers hear the tones and hang up. The humans hear the 'Leave a Message' and have to choose between hanging up immediately or leaving a message. I get to choose whether I feel like calling them back.