This all works fine for the purposes of the telephone system. I mean, at least for a long time, it did. But have you noticed what's up with email lately? It seems that, given an open communications system, people will inevitably develop something called a "cryptocurrency" and badly want to make sure that you get in on something called an "ICO." The general term for this phenomenon is "spam," and the fact that it is only one letter away from "scam" is meaningful as the line between mere unsolicited advertising and outright crime is often razor thin.
In the email system, this problem has been elegantly solved by a system of ad-hoc, inconsistent, often-wrong heuristic classifiers glued to a trainwreck of different cryptographic attestation and policy metadata schemes that still haven't solved the problem. It is, perhaps, no surprise that the phone system is taking a generally similar approach.
The whole STIR/SHAKEN thing first crossed my path a few years ago at The Enterprise. At the time, I wasn't sure what the difficulty was in stopping spam/robo calls and that the Oligarchic Cell Phone Companies were complicit with said calls because it made them money. The actual story, covered in the above article, is much more complicated and nuanced than my own cynical take on it (worth reading, even if it's a bit long). By the time I left The Enterprise, we were starting to support it with our offering (which was “Caller Name ID”—that is, given a phone number, map that back to a name), along with a process that was attempting to classify the originating side of the call as legit or not if the call wasn't attested (that was being done at another department within The Enterprise). If you use a certain Oligarchic Cell Phone Company, and see the name “Potential SPAM” as the caller name, you were using code I worked on.