Or just do the fade-out on the phone's volume control...
and sometimes it's a USB stick or a laptop, and there dozens of kinds of phones that have all sorts of response curves and rates for volume controls, and some devices make noise while you're operating their volume controls, and I have enough to think about without operating some random device, and I already have a lovely set of volume controls on my board that I know and would prefer to use, but, y'know, thanks for your helpful suggestion.