Wonderful idea. The kid can call their friend "Let's meet outside". Then they go outside and (must) leave the phone at home. They use the phone to organize no-phone time together. Might be good for adults too.

For instance my boss couldn't call me while I'm out and about. What you expect me to carry my landline with me?

I know somebody that’s been landline-only since the 70s and only recently got a cell phone that’s strictly used for one specific unavoidable 2FA thing. One of the happiest and most professionally accomplished people I know.