It's not as easy as it sounds. The POTS network was not built with this sort of authentication in mind. Though every major carrier has converted to all-digital

It's not as easy as it sounds. The POTS network was not built with this sort of authentication in mind. Though every major carrier has converted to all-digital backhaul (which still leaves a number of last-mile exceptions here and there), every evolutionary step was built around backwards compatibility. What was never there can't be magicked into existence, though of course it can be strongly encouraged with incentives. Even though mobile-to-mobile and VoIP-to-VoIP calls carry subscriber information under the hood, other call pathways don't or require translation of the information. STIR/SHAKEN are designed to carry this information end-to-end but the need for backwards compatibility and the usual slowness of cross-vendor cooperation leave some gaps.