On landlines, Nomorobo cannot outright block all private/unknown/anonymous robocalls. We can block some, and reporting them does help. Most phone carriers offer their own tools such as "Anonymous Call Block" so check with your phone carrier for additional options.
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Android devices do have a built-in ability to block these type calls.
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iPhones offer no service or application that can. This is a limitation of iOS and Apple has not yet offered developers this ability.