woo_usn_add.php

<?php
//this hook take 3 parameters
// api_used : the api name selected into the SMS Gateway screen
// phone_number : the phone number who will receive the message
// message_to_send : the message that will be sent to the phone number
// you must return 200 if the delivery is successfully or 40X if it fails 
add_filter('woo_usn_send_sms_to_customer', api_used, phone_number, message_to_send);