You can probably stop 99% of those calls by obliterating every business that engages in telephone solicitation.

At my shop, nearly all of the "Unavailable" or "Unkown" calls are sales calls that I don't care to waste my time with. I don't answer them. A few of them, however, are customers calling from their workplaces. Once they start speaking to my answering machine, I pick up the phone and talk to them. Having Caller ID and using it this way saves me a lot of time and frustration.