You can set your phone to ignore calls from unknown phone numbers which are not saved in your address book. When you get a call from an unknown phone number, it will be diverted to your voicemail and is shown as a missed call on the call list.
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Press Silence Unknown Callers.
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Press the indicator next to "Silence Unknown Callers" to turn on the function.
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Slide your finger upwards starting from the bottom of the screen to return to the home screen.