The best thing to do is answer, don't say anything and immediately hang up.  This can simulate a "call block" or invalid number.  DO NOT ANSWER AND DO NOT PRESS A BUTTON SO THEY WILL NOT CALL AGAIN.  They will call anyway and register your number as a valid one in the process.  Before you know it, you will be inundated with scam calls.  These usually originate overseas and use bogus numbers.  

Yes, on 01-21-2014.  No message left.  Tried calling it back.  Rings and rings. No answer and no Voice Mail machine.

I was dumb enough to answer. Girl named Heather said she was with credit card customer services and there were no problems with my cards. I hung up on her.