I suppose it's not that easy. They can use international VOIP providers to route their calls, maybe even run their own, rotating the numbers they use and disguising the actual origin. If that wouldn't work you could probably hire people in other countries to put some agent software on something like a smart phone and use these.
It is that easy. Regardless of what convoluted setup they're using, at some point those calls need to get to the US-based carrier. That is where they can be blocked. The carrier can simply say "we'll stop peering with you if more than 10% of your calls are spam" and the upstream carriers will have to clean up their act (better screen their customers or implement similar terms) or be cut off from being able to reach the US.
The problem is that the US carrier gets paid for terminating the call, regardless of whether it's spam or not. Why would they kill the golden goose?
That works until you land in a situation where the calls are coming from a carrier who can't (or won't) do enough about it, but the carrier has enough legitimate traffic that it will annoy many of your customers to just shut them out. Remember: All the actual companies or carriers you can identify have nothing to do with the actual scammers. They are intermediaries and the scammers will hide between legitimate calls in one way or other.
The problem is bad enough that a us customer might pay extra to not receive those calls. So maybe the phone company can profit off of a service to solve the problem.
