In Canada, we have mandatory Amber Alert notifications that affect all modern cell phones. It is loud so you aren't likely to miss it. These alerts cannot be muted, and require the user to acknowledge receipt or another alert will come shortly thereafter. This helps to catch people during alert time in case their phones are not on, or they're in the shower, or whatever.

I am surprised California and other wobbly places don't use the same system, with the addition of quake and forest fire alerts. Everybody here knows the alert sound instinctively now, so even if you're in public and hear someone else's phone go off, we all know what it is.