iPhone
Screening unknown callers will ask the “unknown caller” for their name and reason for calling. The caller needs to give a short response or, if the caller is a recording and begins to ramble, the call will not go through.
To set this up, go to Settings, Apps and then Phone. Scroll down until you see the category “Screen Unknown Callers.” Select the middle option for the screening to take affect. Silence is another option that will just send the caller to Voicemail and it will only be displayed on the Recent calls list.
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Android
To set this up open the Phone App, touch phone icon, tap three dots in the upper right area, settings, under Call Assist you will find Call Screening. Turn that on and set to unknown callers.