I get scam phone calls with area codes from most of the major metro areas all over the USA although since it's a California area code, it seems most are from somewhere in California.

Your final paragraph is pretty on-point.

> It is somewhat amazing how the telco's have let their core product, voice calls, become nearly worthless by not handling these spam call problems

I have more than once thought about just giving up on having a phone number at all. Unfortunate that isn't an option yet.