They keep calling. And when I pick up the phone they hang up.

who called me

Call back gets 1 ring followed by 30 seconds of silence then this recording, "Thank you for calling.  We're sorry that no one is here to take you call.  Please leave a detailed message with your name, address and phone number and we will call you back.  Thank you and have a great day."  Are you kidding?  How dumb are these people?  Wait, I know this one!  The answer is "too dumb to ever be answered or called."  A company that hides it's identity, never tells you who they are, and asks you to leave this anonymous, unknown caller a DETAILED message including your address and phone number is one of two things:  A telemaggot or a fake debt collector.  BLOCK IT.

They keep calling me when I answer they hang up.