It's a scam.   It's getting more and more widespread.
The way it works is, you receive a call from a number you don't recognize, then when you pick up they hang up, or you might here somebody before hanging up.  Other times they let it ring a couple of times and hang up.
The whole point of the scam is to get you to call them back, at which point a charge is placed on your phone.
Sometimes is just a few cents, other times it could be quite more.  So the simple thing to do is to ignore the call, and add them to your block or ignore list if your phone allows you to do that.  My S3 allows me to add the number to a "Reject List"