Download RoboKiller.
Seriously, it’s the best thing since sliced bread. Not only will it block unwanted calls, it forwards the call to an automated message pretending to be a person. You can make your own message or use one of the 20 precanned messages.
A telemarketer called me and had a 3 minute conversation with a woman and a crying baby. It’s about $20 a year, but oh man, so worth it!