I’ve also been looking at setting up a dumb phone for my kids, but haven’t thought to go the rPi Twilio route.
There’s an existing product that seems to have some mindshare in the space: https://tincan.kids/
Basically $100 for the phone, plus $10/mo to call arbitrary numbers. My main concern here is the company goes belly-up and you’re left with a phone-shaped brick.
Another option I considered was a cheap unlimited SIM coupled with a GSM “desk phone” e.g. https://www.amazon.com/Generic-Wireless-Telephone-Cordless-8...
The problem here is there’s no way to filter inbound calls. The last thing I want is spammers ringing the house phone in the middle of the night.
That Tin Can device has infiltrated (in a good way, what a fun idea!) my oldest kids’ social circle at elementary school. Most of his friends are requesting one for Christmas and we’re planning on getting our kid one as well.
Really hoping the idea lasts, lots of good memories of dialing friends on our landline growing up and chatting casually.
Now if only it could replicate the dread of calling a crush’s house and her dad picking up the phone…