[asciimov]

I'd skip the projects and just buy one of the numerous phones that target this market. They make watches, dumb phones, and smart phones, that are locked down and only allow phoning/texting specific people.

The other option would be to get a real copper phone line.

My reasoning is that a real emergency may include a power outage. Relying on some cool hack isn't going to be helpful when your child is trying to reach out to you when there is no power at home.

[alerighi]

Unfortunately in my country even landline phone lines don't work on power outages, since with the transition to digital only lines (with VDSL) is the router that inside has an ATA to which you connect your landline phone. In fact you have VOIP phone service at your house (in fact you can avoid completely to connect your phone to the router phone socket and use whatever VOIP phone you want, with the correct settings extracted from the provider router).

Even if you provide backup power to the router is not enough, since at the other end, VDSL cabinets in the street, there is no backup power, so if it's not a blackout of only your house but it insist the local transformer, the VDSL cabinet is probably out of power too and thus no line.

The only reliable solution is 4G that is not so reliable since in case of emergency lines get overloaded.

I probably should get an ham radio license to be safe...