code: public boolean sendUnidirectionalResponse(Response response) {
      initErrorInfo();
      LOG.info("Sending unidirectional response: {}", response);
  
      try {
        parent.getSipProvider().sendResponse(response);
        return true;
      } catch (Exception ex) {
        setException(ex);
        setErrorMessage("Exception: " + ex.getClass().getName() + ": " + ex.getMessage());
        setReturnCode(EXCEPTION_ENCOUNTERED);
        LOG.info(this.format());
        return false;
      }
    }
docs: The sendUnidirectionalResponse() method sends out a stateless response message. The response is
  sent out as is.
  
  @param response The response to be sent out.
  @return true if the message was successfully sent, false otherwise.