For the nerdy who might want to set up their own similar system for their kids, and let their kids pick any landline phone they want, you can get an ATA (Analog Telephone Adapter) on eBay for cheap, then connect it to a Raspberry Pi with Asterisk, and any VOIP provider, to make your own PBX. (https://www.littlebytesofpi.com/raspberrypihomephone/)

Can I use Google Voice as the VOIP provider? I read that article and couldn't tell if Voice is used in addition to another VOIP service, like Ooma.

It blows my mind that the current Cisco ATA is $150+.

For the service I am building (havenphone.com) I used Grandstream HT801 with success with voip.ms. I don't love how easy their cloud can take over the device though.