on iOS, the "Silence Unknown Callers" feature has completely eliminated this for me.
If I have a sick relative and explicitly expect an urgent call, I can easily and briefly turn off DND mode.
If the concept of a telephone never existed, and "Phone App" was invented today, it would be considered extremely intrusive and likely not (at least on iOS) pass App Store review. Think about it: Here's an app that allows any random person to cause your device to 1. interrupt whatever foreground app you have running with system-level UI (notification or full-screen takeover), and 2. ring and vibrate your device without your consent. If we weren't already familiar with telephones, we would never accept such an obnoxious app!