I've got a handful of phone numbers, and the amount of spam each gets is quite different, seemingly based on their history. My longstanding gvoice number gets a some, my parents' old landline number gets more. Some numbers that came right from a VOIP provider get basically none.
When I was taking care of my dad in his final months, he had stopped answering the phone because of all the spam. I hooked an FXO ATA to his copper landline and connected it to Asterisk. When the caller ID was on the whitelist, it was allowed to ring. When the caller ID wasn't on the whitelist, Asterisk picked up, played SIT tones and "please try your call again", and made it so the number would be considered whitelisted for the next call (aka greylisting). This worked remarkably well for blocking the spam. And the one time a legit caller wasn't on the whitelist, they dutifully called back (I had fat-fingered my uncle's number in the whitelist, oops).
That is to say, I think there is a good market for telephony products tailored to specific use cases rather than just naively passing through every incoming call. There is no reason the kids' phones have to ring for anything but bona fide calls from their friends during appropriate hours.
(I also dream of setting up an on-hold system of "please continue to wait for your party", for calls from doctor's offices and the like where they're all too happy to drop a voicemail to consider their responsibility over, and then it becomes your responsibility to call them back and wade through multiple levels of phone trees just to put a note in the system for them to call you back again)
Back in the mid-2000s, I used to get calls in the middle of the night, multiple times per night, from fax machines to my fairly new but unlisted number. The ringing would wake me up. I answered a couple of times to try to understand who it is, only to hear the fax tone.
I thought I'd be clever and figure out whose fax machine was calling me, so I could tell them to update their call list. So I installed some fax machine software on my PC and hooked it up to the phone (somehow). Unfortunately, it was all advertising for various scam-like products and services, so I couldn't identify a caller.
Apparently, the number the phone company had given me once belonged to a fax machine. So I called the phone company and changed my number. The calls stopped.
