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.