I have a code that I share with my wife. Normally I answer her calls if at all possible. But if I can't pick-up, for example I'm in an O3, then she can reach me in an emergency by calling 3 times in a row. The same system works for me to call her in case of emergency.
Recently the system worked. Before I went in into an O3 with my boss I put my phone on its vibrate setting and clipped it to my belt. During the meeting I felt 3 separate calls come in one after the other.
I apologized to my boss, checked the caller-id, and answered the call. Indeed it was an emergency. If I had waited until the end of my O3 I would have missed the funeral of a relative, who unexpectedly died.
It is not hard to imagine other "immediate response" personal emergencies -- like children being rushed to the E.R., where you want to act right away even if you are in an "important" business meeting.
I admit that having my phone vibrate during a meeting can be a little distracting, but I think that it is a fair price to pay. And in this case it worked out.
Steve (from Israel)