The penalties for permitting more than X robocalls through in a given period including those originating outside your network should be so high that legitimate

The penalties for permitting more than X robocalls through in a given period including those originating outside your network should be so high that legitimate operators under US jurisdiction are terrified of allowing connections from companies that turn a blind eye to robocall farms—a couple of those stop being able to route calls to the US, and they'll either shape up in a hurry, or continue not being able to route calls to the US, and either way, problem solved.