This might work:
formatting link
It's from Privacy Corps, but it costs $100.
It clams to block "anonymous and unidentified numbers" Been tempted to try it, and then the telemarketers go away, so I forget about it. There is also the configuration problem; how does it block all the phones in the house. Can I mount it where the line comes in, and then tie all the phones to it? They sell "remotes", but that drives the cost up. Verizon has this "iobi" service that appears to have the ability to block unwanted calls. It costs $4.95 per month when added to a "Freedom" package ($7.95 without). More flexibility, it seems, than the Privacy Corps Caller ID.
Lena