I've gotten to where I don't answer calls from area codes I don't recognize. If they don't leave a message, they go on my blocked list. For a while, the phonespammers were using numbers like 000-000-0000, but my VoIP service now lets me block those, too. I use my VoIP service to screen ALL of my calls, and forward only the ones of interest to my cellphone (my cellphone number is not given to *anybody* but my wife).
If you don't have a VoIP service, you can use the new GrandCentral.com (free) to do many of the same things. However, what is really needed is a law similar to the fax law that lets you go after the [***] for enough money to make it interesting.
I received a recorded call on my cell phone from this number that was trying to sell me an extended warranty on my car. I waited for a live sales reptile to answer and I told him that he has no business calling a number that is on the "Do Not Call" list. He didn't care. With all of the telemarketing calls that I have been getting on my cell phone, I am convinced that the "Do Not Call" list is useless.