The problem is with POTS, you dont have that kind of capability in the protocol, even Caller ID cannot be verified. Most network will trust whatever is being sent. It is like SMTP it was designed in era where security was simply not there.
(It's true there are a bunch of important things that PGP has helped solve. Ubiquitous person-to-person secure communication and person-to-service cryptographic authentication are not amongst them. PGP is certainly usefully employed in some niche use cases, but it has failed at pretty much all it's original goals. I can't remember that last time I used it for anything except verifying a software download, and even _that_ use case only applies to a tiny fraction of places that hoist software downloads. My Arch linux installs running pacman and silently checking php signatures for me may be the only time I've had PGP code run in maybe a decade...)
