It may be a surprise to the HN "incentives are the only way to do anything" crowd, but there simply is not an incentive structure that will fix this problem, because nobody knows where the calls are coming from. Not the US, not the telcos, not the countries the calls are coming from,
nobody. You can place the tariff on countries, and then the countries just have to accept the tariff because there's not a damn thing they can do about it as long as scammers can spoof phone numbers.
The converse of this is that it's pretty ridiculous to assume that countries have scammers in them because they're just tolerating them.
Sure, we could rush out a technical solution, but then it would run the risk of being broken rather quickly, and further it's important to not compromise the reliability of the phone system. Too many critical services rely on it. It's worth taking the time to do it right.
Last I heard, the FCC was in talks with telcos to finalize a solution, but I would expect it to take at least another year to implement and deploy it.
Until then, I guess we'll all have to deal with annoying scammers and comments by people who don't understand why spoofing makes their pet incentive idea irrelevant.