calls and hangs up on machine

This phone number calls me a number times per day and hangs up almost always when I answer.  I called it back.  I reached a voice message recording stating, they are a charity fund raising center for a number of charities.????

I hav received calls from this number on a few occasions.  When I answer the phone, there is no response.  The Caller ID only shows, "Chicago, IL"

call me often and leaves no message but hangs on long enough for my machine to pick it up

call me often and leaves no message, sometime call around 8AM in the morning weekend or weekday.

when I answer they don't respond.