According to some new commits that were discovered by XDA-Developers Android P might be bringing a new feature to the stock dialer app. This commit was part of a new set of commits that were added to AOSP from a Sony Engineer.
This new feature would allow you to block all incoming calls from any number that isn’t in your list of contacts. Text strings found in the commits suggest there will be option to block numbers based on the following criteria.
Phone number not in contacts list
Phone number not disclosed by the caller (Private number)
Phone number is from a pay phone
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