You can cut through a surprising amount of spam by just reading the voicemail after letting an unknown number ring. If they didn't bother to leave one it was either not at all important or the vast majority of spam calls. If they did leave a voicemail you can generally tell whether it seems even remotely legitimate and, if it does, call back at the number. E.g. if you have a kid and the school calls they will leave something generic like "It's so and so elementary calling for <your name>..." you call the school's main number back and say "so and so called looking for me" and get directed to them.
OTOH it seems if you ever answer these even once you're no longer a random number to try you're immediately marked as a real person who can be tricked into answering at least some calls and will never hear the end of the endless ringing.
This is genuinely pretty shocking to me. Would e.g. the person calling from the school about your kid getting hurt during recess just hang up and assume you'll call back immediately during the workday on the basis you had a missed call?
If you mean for people you have relations with (friends/family/coworkers) most here gave up memorizing numbers a long time ago and none of those would show as an unknown number. This is common enough here in the US that the iPhone has a built in feature to only ring for known contacts and for unknowns only notify voicemails.
Does anyone still has voicemail enabled in the first place? Can't remember the last time I was connected to one. Can't remember last time I had one attached to my number either.
UK here, called my phone company many many years ago and had them disable voicemail. It's now impossible to leave me a voicemail. It's fantastic. In all those years I've never (that I know of) missed an important call, people call back if they need to.
If you don't have voicemail, they will send you an SMS. Parent's main point IMO was that if the call was really something important, the person on the other end will leave a message of some sort.
The person on the other end doesn't know if you are making yourself deliberately hard to contact, or if you are genuinely unable to answer the phone call.
This strategy works fine in my experience. Most important calls I get are from people already in my address book. If I'm in a job hunting phase, I'll answer calls from numbers I don't recognize during time frames when I am expecting them (e.g. the policy is relaxed for the duration of a job hunt). But, outside of that, it's hard to imagine a call from someone I don't know that is so important I can't let it go to voicemail, but so unimportant that the caller will deign not to leave a voicemail to let me know this is really important.
Wait a second, do you pick up any call you receive? IME Australia is one of the worst places with regards to spam calls (by far the worst out of all countries I lived in), if I allow calls from numbers not already in my contacts, I get an average of around 10 spam calls per week. If it's something important like my daughter's school or strata or the lawyer, they do leave voicemails.
Honestly pre-COVID I got spam calls maybe once a quarter. After giving out my number every second time I left the house, that number is more like a few times a week.
I would say I am pretty privacy conscious (when possible), so this may not be the average person's experience.
The problem with this strategy is that sooner or later, the spammers will find your "trusted" number. After all, phone numbers are easily enumerable...
That's surprising to me, almost everyone I know will leave voicemail. Recruiters especially, which tends to be myst of my legitimate phone calls TBH!
Though with people of my age "cold calling" isn't all that common, typically you text the person first to make sure they are available and keen on a call. Unless it's an emergency of some sort, of course.
OTOH it seems if you ever answer these even once you're no longer a random number to try you're immediately marked as a real person who can be tricked into answering at least some calls and will never hear the end of the endless ringing.