To reach the most (or least) called/ calling people in the book
Why not call the people who seem to think that using the phone is important, and don’t bother calling people who don’t use the phone frequently? The frequent callers/ callees are probably the people who react best on beeing called, right?
My experience is that some people like (depend) on using the phone do get things done. Some people are very important, so they get many phonecalls (this might be even enhanced through good replies).
You could also attach other conclusions on beeing called frequently, or calling frequently. For example for companies: bad service, bad product, good service, good network (of people), or for people ((a)social, good network, stalker.