Silence Unknown Callers
Unknown callers are obnoxious, so it is time that they are silenced. To do this on iOS, go to Settings > Phone > Silence Unknown Callers.
Once that’s turned on, any number that’s not in your contacts, Siri suggestions, or recently called list will immediately go to voicemail. You won’t even have to deal with ignoring the call yourself because your phone does it for you. Cool, right?
You’ll still receive a silent notification that tells you a call has been silenced and have the option to view the caller in the Phone app under Recents.