I didn't answer.  They didn't leave a message.

Unknown to me number. I did not answer and they left no message.

It's the free cruise survey scam.  Call back gets, "Thanks for calling.  The survey you were called for is now finished."    There is no survey just some nonsense questions.  You'll be offered a free something for completing the survey then asked for your credit card number.  It's a SCAM.  Block it. Here's a news report on it:
https://www.youtube.com/watch?v=I4tY1kz6lZ8
http://scam-detector.com/travel-scams/free-cruise
http://protectingyourpocket.blog.palmbeachpos ... n-package-scam/
http://www.floridatrend.com/article/20232/flo ... -vacation-scams