Unfortunately, in North America at least, it seems the phone scammers and number spoofers are running rampant. There is no way to escape them. They do mass broadcast robo-calls looking for any hint of a live number, and continue the 'sucker hunt' from there.

As a result, I assume that every call is a scam call -- until I have evidence to the contrary.

Yeah, in North America we're pretty much at the mercy of spam and shady businesses. There are some laws against spam and robocalls but almost no resources are devoted to enforcing them and there are tons of excuses. Unsurprisingly, politicians created exemptions for political robocalls. I expect that algorithms find numbers and if you ever answer one they'll never stop. I get between a couple and a half dozen such calls per day. About the only relief has been to set my phone to "Do Not Disturb" on the setting that allows calls only from my contact list.

Not giving out your number doesn't prevent someone from being able to dial it. Many spammers simply automate starting at "0000“ and increment calling each until someone answers. Once you answer they know it's a real person.

Something I've found to be somewhat effective, when I suspect a spammer, is to answer and not say anything. If it's a robot caller it waits to hear someone and then starts the recording or kicks you over to a person. If it hears nothing it gives up and marks the number as a non human number and quits calling. If it's a legitimate caller they'll eventually say "hello" and then you can answer. Although it still may be a spammer.

It's not 100% but I've significantly reduced the robo callers doing this. Your mileage may vary.

That doesn't reduce it for me, their autodailer just keeps calling back. What I found works best is to answer it, say hello or press a button, then mute the phone and sit it down. It will connect to a person and then that person's time gets wasted saying "hello? hello?". Those actually waste the scam companies time so they will be more motivated to remove you from their list.