Greylisting is "method of defending e-mail users against spam. A mail transfer agent (MTA) using greylisting will "temporarily reject" any email from a sender it does not recognize. If the mail is legitimate, the originating server will try again and the email is accepted. If the mail is from a spammer it will probably not be retried since a spammer goes through thousands of email addresses and can not afford the time delay to retry."
Why not apply a similar technique to your telephone? It could be done without much effort in this day of caller-id. Imagine if you had a device that, when plugged into your telephone line, would allow you to automatically answer the phone for unknown numbers and give the caller a message. The message could be generic or it could be instructions to do something specific. You could be program the device with a list of white listed (allowed) telephone numbers as well as blacklisted (denied) numbers. The device could give a different message based on whether the number is in the blacklist or simply unknown. If the calling number is on the white list the device does nothing, allowing standard telephonic devices to continue operating.