I found picking up then hanging right up very fast messes them up. I usually don't get a repeat call.
What TelcoKitty said is right. Answering, saying hello or calling back these numbers actually makes you get more calls. Your number is marked as a working number for every telemarketer in the World.
Looks like they really are calling everyone today! Saw a missed call with no voicemail around 4:00. I didn't call them back. Based on these comments, it looks like yet another scam.