How to limit the Phone app usage, how does Communication Limits - Contacts work?

I'd really need to limit the Phone app usage on my child's iPhone. Her friend calls her all the time and it's really taking too much of both of their time. Great she has friends, but trust me, this is too much.

In my device's Settings - Family - child's account etc. there's Communication Limits and Contacts. Should it be possible to access the child's Contacts on her device & iCloud here and set limits to specific people in Contacts?

Surprisingly you can't set limits to the Phone app as to all other apps, so what would be the way to limit certain people's calls, or more specifically when and how long they're allowed to call? (The answer is of course "hide the gadget..")