There are users that uses the RingCentral Phone embedded with their CRM or other calling platform. Thus, selecting the default calling for app to "RingCentral this app" is not a choice for them, and the popup that says "An incoming call is being directed to the RingCentral Phone app. Please review your settings if you want to see incoming calls in this app." cannot be removed permanently which some users find annoying.
There should be an option to remove these banners permanently that can be a setting on the app or an admin setting.