> but refuse to answer calls from numbers that aren't in their address book.
I do that. If I don't know the number and that person really wants to talk to me, there is my answer machine where he/she can leave a message. I will never answer a call from a number I do not know, whatever the reason. If it is urgent, the person has to leave a message, so I can call him/her back. Why ? because I get spammed X times a day with numbers from unknown origins. The worst is masked numbers off course.