nothing, no way, ain't no free anything.
i back rickpolitic on this one.
Set your phone options to block that number (if it appears in received calls) or set your phone to block anonymous calls like "private caller", "out of area", etc
look on your phone under recived calls and it should be there.
If you don't know the number, don't answer it. Those idiot just have too much time on their hand.
Ignore it. It's a prank.