Calls on my unlisted mobile at all hours

Is there any way to stop these people?  How dare they do this and why is there no protection against them?  I have never answered a call as I never do with an unknown number but it still intrudes on my life--driving, shopping, sleeping, or just busy I can't always be running to the phone.  How do they even get peoples' mobile numbers?  Our sheriff has warned against them but seems unable to stop them.