Here is our step-by-step guide on how to block annoying calls from unknown (withheld, restricted, private) phone numbers on Android. We will show how to do this on Samsung Galaxy Ace III GT-S7272 under Android 4.2.2. The steps to block calls from unknown numbers on Android 5 and 6 are exactly the same.
How to block calls from unknown/restricted/withheld numbers on Android 4, 5, 6
Select My device and tap on Call category:
Choose Call rejection:
Check that the black list function is enabled. (The Auto reject numbers should be chosen in Auto reject mode setting).
Then enter the Auto reject list setting:
Check the flag Unknown to add all unknown callers to black list:
Henceforth, all incoming calls from restricted (unknown, withheld) numbers would be automatically rejected.