private function buildSmsPayload( $type, $to, $templateId, array $personalisation, $reference, $smsSenderId = NULL ){

        $payload = $this->buildPayload( $type, $to, $templateId, $personalisation, $reference );

        if ( isset($smsSenderId) && $smsSenderId != '' ) {
            $payload['sms_sender_id'] = $smsSenderId;
        }

        return $payload;

    }