I only recently got "Caller ID" on my phone, and it helps me to avoid calls from telemarketers, charities, and politicians.
If I see that the ID says something like PUB INT (for public interest), I ignore the call. I do the same any time that the ID shows UNKNOWN CALLER. Once in a while a number appears that might be from the same area code as a relative or friend, and I may pick up the phone by mistake. Here's what happened yesterday: The phone rang, I thought it was a relative calling, and I answered. As I said "Hello" I heard the sounds of a "boiler room"; that is, I could hear that a large number of people were speaking in the background as the caller asked "Mr. NorthAmerican?" Then I said "Hello?" and "Hello?" again as the caller spoke my name twice more. Then I hung up. He would have thought that he had a bad connection.
Other times I use a method somewhat like the one VinnyD described earlier: I just say, in as few words as are necessary, that I don't have time to speak, and if they start to object, I hang up.