code: public void hangUp() throws Exception {
          final Map<String, Object> params = new HashMap<String, Object>();
          params.put("state", "completed");
  
          final String uri = getUri();
          client.post(uri, params);
  
          final JSONObject jsonObject = toJSONObject(client.get(uri, null));
          updateProperties(jsonObject);
      }
docs: Hang up a phone call.
  
  @throws IOException unexpected error.