I purchased an app for my iPhone called Call Bliss.  Anyone not in my address book goes directly to voice mail without my phone ringing, and I can see who tried to call.  If the caller leaves no message, I don't care, and I don't even hear it ring.  It gives me the opportunity to google the phone number that tried to call to see if I "really" want to call back.  I love it.  It cost only $1.99.  There are other similar products out there for non-Apple devices as well.