I realized today that I'm a phone number screener. If someone calls my mobile or home phone, I make sure that I recognize the phone number prior to answering. I'll think twice about answering if a number looks familiar but doesn't necessarily trigger recognition. I won't even think about answering a "Private Caller" or "Unknown." I suspect that I'm part of the majority when it comes to filtering calls.
What's most interesting about this now-dominant phenomenon is the speed at which it's become a nearly ubiquitous human habit. Thirty years ago, every call was a "Private Caller." Twenty years ago, few home phones had Caller ID and mobile phones weren't even in the picture. Ten years ago, the habits had sprouted with built-in Caller ID and some mobile phones. Today, my friends don't answer my calls unless my number is in their phone.
Many claim that the digital-communication revolution (both Internet and mobile) has fostered only open and free communication. Here is a prime example of an opposite trend primarily driven by the same technology.