I work at RoboKiller.com
I've always thought the way to battle this problem is to waste the callers time. The longer you keep them on the phone, the more expensive and less profitable their robocall operation becomes. With enough people doing this, it could slow them down.
At RoboKiller we intercept the call so it doesn't ring your phone or go to voicemail. Instead what happens is we answer the call on our end and proceed to waste the callers time by tricking them into thinking they're talking to a human. Its really funny actually. We send you the recording when we're finished.
A couple weeks ago we reached a point where the top robocallers caught on to what we were doing. It was a proud moment for all of us. You can listen for yourself...