Light/Heat Sensitive Cell Phone Ring Volume Attenuator

What I typically hear when a fellow bus passenger's cell phone goes off is first a few very loud rings, and then excruciating *even louder* ring, as their bag is now unzipped and their phone now in hand, but before they actually open it and press the "connect" button.

Once the person is holding their phone, or even rummaging in their bag for it, there's really no need for such loud alerting anymore. Hence I propose the phone have sensors for changes in ambient light level or the heat of one's hand, that determine that the bag (or pocket) is now open, and reduce the ringing volume.

Once the light/heat trigger is activated, the ringing should reduce to a quiet tone, just loud enough so the answerer can tell if there's still time to answer it.

Thank you.