Number spoofing is not a solved problem because some carriers, which appear legitimate in all other respects, make a business out of routing your traffic over TDM trunks that don't support caller ID verification, and will claim it's extremely expensive to upgrade these to VOIP.

Fuck 'em? That's not a insurmountable problem in the slightest. Google or Apple could probably solve this problem themselves by simply not ringing the phone for any call that doesn't meet ID verification.