They have called me several times today. I tried to call it back and got a recording that says "The number you have dialed is not in service"

My question is, how did they get my unlisted phone number?  And,  How do you get them to stop calling?

This same number keeps callin me but leaves no message either.

This number 607-270-0533 has called my home phone 5 times in one day.  They never leave a message.  The ID shows Ithaca, NY.