Blocking "Restricted" Calls on Smartphone

I've read this before, and I always wonder what you do when you get a call from a number you don't recognize, and it's from someone who borrowed a phone to call you for one reason or another. We've had that happen with our great-granddaughter.

FWIW we get very few, if any, spam calls on our cell phones, but I'd hate to think that someone needed to get a hold of me and I ignored it because it simply wasn't on my contact list. I guess if they really need help, they'll call back...oh yeah, and get ignored again. :)