Imagine if all the people you wanted to call did the same!
I think MOST people do this. I keep my phone in the car 90% of the time no on my person. At home I have an answering machine and I don't pick up the phone if it rings. To many telemarketers and junk calls.
What I'd really like is a good, non spoof-able caller-ID system. Then I would want a phone that routes calls based in caller-ID. The people on my short list would cause the phone to ring others would go to voice mail directly and those with no caller-ID would get a message telling them to enable caller-ID and redial. In addition the system would now my schedule and re-route calls to the office or home or cell based on time of day, day of week and have a voice mail to e-mail gateway too. That last part was great because voice message end up in Google's gmail as WAV files that you can click on
I actually built an Asterisk based PBX and had it do all of the above. But it was not worth the hassle to maintain it. (But is was a geat system. See here for more info http://www.voip-info.org/wiki/index.php?page=Asterisk )