I want this as my voicemail message:

This phone is set to vibrate. If this is really important [1] dial '73' to make the phone ring. Otherwise, please leave a message.

I leave my phone set on vibrate and often ignore it: just because I have a box in my pocket doesn't mean that people elsewhere should take priority over people in the same room. In person, if B comes up while I'm talking to A they will politely wait until A is done. If it's urgent, they can say "excuse me, but this is really important", and seize priority. Phones should have this too. It wouldn't have to go into full ringing: a brief sound following a vibrating ring that I had ignored would tell me that someone had claimed urgency.

(Would people claim urgency excessively? I don't think so, but if they did in practice I could make this available only to people I trusted not to abuse it.)

This is part of my general retrogrouch outlook.

[1] And possibly: "... and you're willing to pay $1 if I disagree ..."