docs: public boolean addHangupListener(CallHangupListener listener)
      {
          boolean callStillUp = true;
  
          if ((this._originatingParty != null && this._originatingParty.isLive())
                  || (this._acceptingParty != null && this._acceptingParty.isLive())
                  || (this._transferTargetParty != null && this._transferTargetParty.isLive()))
              this._hangupListeners.add(listener);
          else
              callStillUp = false;
          return callStillUp;
  
      }