Are you asking about blocking private numbers and unknown numbers?
Go to Settings > Phone > Blocked. Add the unknown caller the same way it shows up in your recent calls without adding any number. For example if the caller shows up as "Unknown Number", then add Unknown Number in your block list without assigning it a telephone number.
You are right for cases whereby "Unknown Number" is showing and you can add that as a caller and even give it a name on your iPhone however what I'm asking about is that "Scammer" who gets through to you and nothing shows on the caller number and in the iPhone caller ID it shows as Unknown Caller with blank info without even a city location... Therefore you have nothing to hang your hat on to "ID" anything to block....
It's a cute and annoying trick they're using and I have to blow them out of the network...