What would happen if the American companies just started playing hardball? For example, what if they required that contracts with foreign telephone companies had a fee based on scams detected as coming through and just refused to work with any company that wouldn't agree to those terms? I have no idea if that exact idea would make sense or even be possible to implement, but my thinking is that in many cases the American companies might be able to afford to lose the business more than their international partners, so playing a game of chicken might be worthwhile. In other words, just because American telecoms might not be able to do anything from a technical perspective doesn't mean that they don't have social or business pressure they could apply if they actually wanted to solve the problem. Ultimately, it seems like the American companies haven't done anything because they don't really have any business incentive to; as long as they aren't held culpable legally and don't lose sales due to the PR hit, trying to use their influence to solve this is purely a risk from a financial perspective.
I don't at all subscribe to the Friedman perspective that businesses are ethically obligated to maximize profits at the expense of literally everything else, but I do think that it's a reasonable description of the way businesses trend toward acting; an entity structurally designed for making profits will inevitably end up acting in a way that only considers that measure unless the incentives are changed, which is precisely why we _do_ need to regulate things to curtail business behavior that's a net bad for society as a whole by making it unprofitable, whether that's through direct fines or making business liable either civilly or criminally for their actions.