I get these calls also. There's never anyone there and they don't leave a message. For me, I just unplug the telephone in the evening. My friends and family have my cell number also. Other than that, just block the number if you can.
I just picked it up, It's an automated survey about upcoming elections. 1st question was: How likely are you to vote in the 2012 elections? I'm going to assume that once you take the survey then they'll stop calling.