That's interesting, I come to the exact opposite conclusion (I am admittedly not a telephone network expert, maybe I am missing some important things).

But I don't see why having voice conversations over the internet is the issue. The issue is these systems keeping compatibility with old telephone networks which prevents solving these problems. If we dropped that compatibility requirement we could require a certificate authority and be able to verify callers and use end-to-end encryption, just like with https.

Or maybe better would be a compromise where all these systems can still fall back to unencrypted/unsigned connections, but on any modern phone or cell phone it would show a big insecure warning like modern browsers do for http.