There is no mitzva to answer the phone. Telemarketer is banking on your curiosity. Worse, when you answer, your number is tagged in their database as a number with a human that answers and you will be called again. Best thing to do is pick up phone but do not say a word, they use voice recognition software to flag live respondent, this is why there is a delay After you pick up and human gets on line. After a few times of “dead air”, your number is flagged as one not to call back for a while.
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