"There’s at least one piece of data on every call that the network operator has—-the identity of the prior network hop."

I'm curious - is this available in a call passed through twilio ?

I would like to log, or pass, the prior network hop using twiml ... or a twilio function, I guess ...

From my best interpretation of your comment you seem to think VoIP is far more "carrier peer to carrier peer" than it is in practice. As a value added telecom provider I have a few upstream carriers but it's not as if I receive calls from every carrier. Just like the Internet, my upstream carriers are my access points to the PSTN, so all of my SIP invites come from either my customers or my know upstream carriers. Those carriers are effectively my ISPs of the PSTN. I chose them and I pay them for service.

Distrusting one of them and the calls that come from them is something of a non-sequitur since you chose them and intentionally became one of their customers and pay them to receive those calls.

TL;DR This isn't like email or IP traffic where you're interacting with other IPs in a purely untrusted network model. Merely analyzing the previous hop doesn't tell us anything valuable.

That’s the peril of being a leaf node. The same thing was true in the early days of email. An end user, or in your case something close to it, couldn’t blacklist his upstream—-as you say that would be ridiculous. But your upstream (or possibly their upstream) does have true peers that they could blacklist for bad behavior. It might mean there customers couldn’t receive calls from entire countries, but that’s a trade-off I, and I assume plenty of other, would be happy to make and even pay extra for.

This assumes that is that there isn’t some kind of international treaty that forbids doing so. The existence of such has been claimed from time to time in similar discussions but I have yet to see an actual citation.