They are trying to get you to say "Yes" or "OK" so they can manipulate the response and use it to steal your identity such as online loans etc. Just hang up. You may report the number, too.
How about this rule of thumb: If you don't know the number DON'T ANSWER IT. If its important they will leave a message. Many of these calls don't leave messages and some have left spammy prerecorded messages about loans, debt reconciliation, free vacations, and other prerecorded messages DESIGNED TO RUIN YOUR LIFE.
Don't be stupid like "D. Heath" up there. When you answer you open yourself up to everything. I barely receive these types of calls but when I do I leave them alone. Then I coe here to find out how many didn't do what I did.