In two words - Assistive Access. Or Senior Mode, colloquially. So I guess that's four words.
In Set up Assistive Access on iPhone Apple says it is designed for people with cognitive difficulties, but it is also useful to simplify the phone for anyone who does not need the full power of an iPhone or, as in your case, if you want to minimise the exposure to a person to the problems they may encounter such as scammers.
Perhaps this is not what you seek, but this YouTube video is one of the better descriptions on this setting and you can see how the Calls app functions for making calls and for receiving calls - it can be set to receive calls from selected contacts, all contacts, or anyone.
The Calls app is discussed and show at about 8 minutes and 10 seconds into the video.