Nomorobo (nomorobo.com)

It's about time that somebody built something like this. Nomorobo maintains a robocaller blacklist and uses the simultaneous ring feature of your phone provider. When the call is from a blacklisted number, they answer and immediately hang up.

Here's the article where I found out about it: http://www.today.com/money/free-service-proves-it-can-block-....