This number calls my business several times a day and typically doesn't say anything. Today when I answered there was a lot of background noise and the male voice said "hello?" I replied "hello, (our company name), how can I help you?" they replied "f%$k you" then hung up.
Try not answering unknown calls-they only continue to call because you continue to answer. Also do not call them back-by doing that you are probably getting more calls.