You're forgetting that 1) carriers have near-zero engineering capability to fix this and 2) don't actually have an incentive to prevent spam/fraud because fraud

You're forgetting that 1) carriers have near-zero engineering capability to fix this and 2) don't actually have an incentive to prevent spam/fraud because fraudulent traffic still pays them money. This is not about stopping fraud, this is about preventing "grey routes" that do arbitrage around tariffs and bypass carriers' outdated business model.