This number calls every evening, sometimes more than once.  There is no caller id information.  I have answered several times, but there is never anyone on the other end, real or computerized.  They never leave a message, and when I've tried calling them back, I get a recorded message saying that the number is "invalid".  Since they leave no information or any way for me to get information from them, I am completely baffled as to why they even bother to call.