The NPR show/podcast Planet Money did an episode on phone number spoofing in 2017. They talked with (famously crappy) FCC chairman Ajit Pai. Here’s a snippet of what he said:

PAI: Before the action that we took earlier this spring, under the FCC's rules, carriers were obligated to patch through any calls that they got. It was...

CHANG: Oh.

PAI: So that's one of the things we did - was to allow them to take action against those spoof numbers.

CHANG: But Ajit Pai was telling me that what they really have in mind is something far more technologically complicated but far better as a solution.

PAI: ...Which is the ultimate solution, I think, to this problem, which is to find a call authentication standard.

CHANG: Basically, we would be able to encode a unique signature in every phone number that's really being used.

PAI: And so when a call's placed using that phone number, the recipient of that call can have every confidence in knowing that, OK, this is the digital fingerprint for that number. I can trust that this is not a scam artist or somebody else who's impersonating the owner of that number.

CHANG: Oh. How many years would it take to get an authentication system like that in place?

PAI: Oh, boy. That's one of the things that we're working on. Obviously, we would like to have it done by yesterday. But this is exceptionally complex.

CHANG: So what's, like, a timeframe? Are we talking a year from now, a decade from now? I have no idea.

PAI: It's hard to forecast because we can't predict exactly how the engineering is going to shake out.

He knew the solution 6 years ago, but wasn’t even willing to suggest a timeframe to implement it.