This feels like it would be useful as a general protocol. A signed app on your phone, can register some special endpoint into a registry that responds with if the user logged into the app is being called. Put some button in the phone interface for “Verify”, which pings all of these endpoints, and either shows a logo and a success message, or an unknown caller warning.

You wouldn’t want it to ping these services on every call, so it would have to be an action you just train people to do.

Better than hiding it in an app though, this should be an OS concern somehow.