What's the difference between an 'unkown' caller and 'no caller ID'?

This unknown number keeps ringing me, and i thought if someone blocked their numbers (on iphones) it comes up as no caller ID so i'm not sure if it is still a blocked number or a number that can't come trhough.
But when i answer nobody speaks and they've rang one of my friends so its not a scam phone call

Added (1). also they don't speak at all so if it was a company of some sort they must of spoken so its probably a prank call but i wanna know the difference to make sure.

Unknown is usually just a problem with the phone company and no ID is usually a burner phone. It is probably a collection agency calling. The use unscrupulous methods to contact people.