Of course, if you don't answer your cell phone it goes to voice mail and plays your pre-recorded message.
But what we need are phones which can answer and play a pre-recorded message for a short time. In particular a message of the form, "Hold on, I'm in a meeting and must keep silent. However, I'm walking out of the meeting right now while you hear this recording, and in a few seconds I'll be able to talk to you. Hold on... Still walking..."
This could be a special answer button on a phone (with the carrier doing nothing) or you could just press a number button (DTMF) or other button right after answer and the cell carrier could receive that and start playing the audio to the caller until you press another button or simply start speaking at full volume into the mic. This latter system would work with any phone, and you could choose from several options to play, including "Hold on, I'm actively driving" and so on.
At a recent conference they asked people not just to put phones on vibrate, but to turn them off unless you're a doctor-on-call. They declared that people getting up (and often briefly talking) was becoming too much of a disruption in meetings. A feature like this could be some of a stopgap.
It could also be implemented in a headset, particularly a bluetooth one, so you could use it with multiple phones.