Phishing scumbags trying to see if a number is legit so they can sell it to telemarketers.  There is never anyone on the other end, just a computer that registers either an "answer" or "no answer."  Usually they call in the wee hours of the morning to businesses.  Most every business has a welcome message for after hours to leave a message so we can gt back to you.  Bingo, the next day those businesses will probably receive 2 or 3 phone calls to update their free Google listing.  347 area code seems to be notorious for this sort of telescam, ergo that area code is blocked on our phone system.