How to add custom sms gateway to Ultimate SMS Notifications for WooCommerce ?

add-custom-sms-gateway.php

<?php

// Use this filter to hide the default API settings.
add_filter('woo_usn_display_sms_api', '__return__false');

// Add your fields for filling SMS Gateways API Keys.
add_action('woo_usn_options_before_sms_api_fields', your_function_here(){
    // display your fields here and make sure to add button to save the settings
});

// Save your SMS credentials .
add_action('woo_usn_save_sms_gateway_credentials', your_function_here( $api_choosed, $data ) {
    // save into the database your api keys for future use when sending sms notifications.
}, 99, 2);

add_filter('woo_usn_send_sms_to_customer', your_function_here( $status_code, $apiused, $phone_number, $message_to_send  ) {
    // this hook allows you to send sms using custom sms gateway.

    // @status_code : the status code by default is 400
    // @apiused : the hook will provide the sms gateways used for sending the sms.
    // @phone_number : the customer phone that will receive the sms, by default, the plugin already sanitize the phone number,
    // @message_to_send : the message you are going to send to your customer

    return $status_code; // you must return a code, 200 if the sms is sent or 400 if there is an error.
}, 99, 4);