> Recently a local news station in Maine reported a story of some middle schoolers calling their friends with landline telephones.

This reflects on another problem: the sorry state of journalism and willingness to turn press releases into news. That story ran in a wide variety of media outlets, and a Google News search of "children landline phones" turns up a bunch of these.

It turns out that these articles were really ads for "Tin Can," a VoIP phone for kids. Not really a landline at all, it's seriously nerfed, and I'd assume that if it's SIP, it's locked to their service, or else it's their own proprietary protocol. Not really a surprise, given that real landlines are almost extinct, and expensive where available.