[templar]

They have NO right to intrude on my life as they try to do.

You can put up a No Trespasing/No Soliciting sign on your gate or door, but your telephone is nearly defenseless.

[RobRoy]

I pick up the phone and say hello. If I get no response within a reasonable time (about half a second), I hang up.

Having an unlisted number helps a lot but is not foolproof. If it wasn't for my need for the mainframe connection, I would not even have a land line.