If we want authentication, public-private key pairs are a great idea.

A trusted network is fine, but are the telephone networks trusted? Can't get them to implement simple features in our interests, doesn't seem very much like a trusted third party to me. Meanwhile, OTT services work. Some let you whitelist contacts. Killer features like that will push everyone off POTS eventually anyway.

Inertia and legacy will keep holdouts using POTS, just like people still use fax machines, but they're mostly irrelevant.