I recently got divorced, so there is a phone at each house in case they want to reach out to the other parent directly. Ex and I did not want the kids to feel their right to reach the other parent needed to ask for permission

Family has Softphone in their mobiles, so the full family is a speed dial away.

I also whitelist numbers they can dial out.

So far it’s working like a charm, they love it.

The thing for me that has really unlocked voice-based socializing has been the 12 year old jumping on Discord with his buddies from school. I feel like this mirrors well how I myself chat with my adult male friends—it's rarely in the context of just "a call" but rather while doing another activity. So when I see him joking around with them while they play Minecraft or whatever, that feels like it's a reasonable pattern for how to sustain friendships.

e.g. corded phone -> cordless phone -> mobile phone -> smartphone

or

records -> tape -> CD -> MP3 -> cloud

or

Atari -> NES -> SNES -> N64 -> Gamecube -> Wii -> Switch 2

If I build it, I can control the full feature set and explain to him how it worked and he'll get the 'cool' factor too. With the raspberry pi I have lying around at home, it doesn't sound impossible!