My kids (12-year-old boy, 7-year-old girl) recently got Tin Can phones, as did several of their friends, and absolutely love them.

One note: you can authorize regular phone numbers for them to be able to call, but only if you pay the subscription ($10/month I think? We didn't do this...)

I know I could build the same thing out of esp32's but it would be a big hassle, and I'd have to build one for all their friends too!