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Calls from blocked numbers are scary, so you'd better not answer.
Literal
Blocked-number [topic-は] scary [because-から] not-answering direction [subject-が] good [assertion-よ].
Practical advice. 非通知 (ひつうち, 'withheld number / caller ID blocked') is a phone-specific compound: 非 (non) + 通知 (notification). 出ない方がいい ('it's better not to answer') uses ~ない方がいい, the negative recommendation pattern: 'better not to do X.' よ at the end adds gentle assertion. 恐い (こわい, 'scary') is the casual spelling variant of 怖い.