This company calls me constantly. I told them I am not interested and now they call and hang up every time. How do you make it stop?

Have been receiving a call a day from this number for about the last week.  Normally I haven't been answering, just unplugging the phone for about a minute to stop the ringing.  Decided to answer on latest call and I mumbled "hello" quietly.  No response, no noise in the background.  After about twenty seconds, it disconnected.  Caller ID read "Warranty."