I, too, find myself wondering why this seems to be such an intractable problem. Maybe it's just misaligned incentives? That is, the phone companies really only care as much as they need to in order to prevent you leaving for another phone company.

From a technical perspective, it doesn't seem to be _that_ difficult: it seems like KYC but for anyone who wants automated access to telephone networks. I know there are some existing efforts there that are more technically comprehensive than that (SHAKEN/STIR), but I don't know where they're at in terms of adoption/rollout.