International carriers still interconnect with a us carrier to deliver the calls. Spam is really easy to detect. Short call durations and lots of different call to numbers.

Even just giving origin information would help, if the local end of the interconnect ensured calls show the international dial-code as a minimum then that would help; instead we get (in UK) international calls (India, primarily) with caller ID showing a local number (sometimes your own number, I think) ... the interconnect certainly shouldn't be facilitating that.

That's easier said than done.

SS7 its the primarily utilized protocol stack nowadays. The PTSN (Public Telephone Switching Network) and the Internet have been combined for a while now.

The problematic part of SS7 is it allows for setting arbitrary unauthenticated origination info in the rough equivalent of a "From" field in the initiation process.

There are attempts ongoing to attempt to implement a "Web of Trust" layer a la TLS on in front of the preexisting telephony infrastructure in the form of STIR/SHAKEN, but it'll have some of the same warts that current "Web of Trust" implementations now currently suffer.

It was my understanding that in PSTN at country-level boundaries the operators choose to forward calls, and do billing and such recording: at that point they decide whether to put the call to the local country network or not. If the call has meta-data as if its come from the local country then they could drop the call request, but they establish the voice call [because call senders pay more than call receivers, presumably].

Is that incorrect?

https://en.m.wikipedia.org/wiki/Signalling_System_No._7