code: public boolean addPhoneCalledListener(PhoneCalledListener listener) {
  		synchronized (this) {
  			if (callListeners.contains(listener)) {
  				return true;
  			}
  			return callListeners.add(listener);
  		}
  	}
docs: Adds Phone Call Listener<br>
  This function is called when the device has a phone call
  
  @param listener
  Listener which implements IPhoneCalledListener
  @return true if already that listener is registered; false otherwise