private function transferSoap(Transfer $transfer, VendorInterface $vendor = null)
    {
        if (!$transfer->getEntity()) {
            $transfer->setEntity($this->entity);
        }
        $parameters = $transfer->mergeIntoParameters();
        if ($vendor) {
            $parameters = $this->mergeSubAccountParameters($vendor);
        }
        $result = $this->callSoap('direct', $parameters);

        return $result['transactionId'];
    }