[AStonesThrow]

In the realm of Caller ID, a phone number may be "PRIVATE" (or "WITHHELD") or "UNKNOWN". An "UNKNOWN" Caller ID cannot display any name nor any number, because... they are not known to the switch.

Therefore, an unknown number that can be blocked/ignored by your phone or the app is one that doesn't support Caller ID's name or number functions. It doesn't have anything to do with who's in your Contacts app, because of course those consist of known names and known numbers.

[conductr]

There is a defined type “Unknown” which I think you’re describing but it’s Not exactly how the iOS feature works. It says let’s through those in your contacts or who you’ve had recent conversations with and Siri suggestions. It’s basically a dumb proxy for letting through people you might actually want to talk with. Except sometimes you don’t know who/where/when those calls are coming from and I haven’t spoken to them before.