WordPress SMS API integration without plugin error

I would suggest using the HTTP API that WordPress provides for this purpose. Take a look at the GETting data from an API section for more details on how to get the data from your URL.

To get started, try:

• Navigate to your wp-config.php file, and set the WP_DEBUG and WP_DEBUG_LOG constants to true (do NOT do this on the production server of your site).

• Next, replace your code with the following:

  function mysite_woocommerce_order_status_processing( $order_id ) {
  
      $billing_phone = "0729424391"; // TODO Sanitize this input
      $message = "yep"; // TODO Sanitize this input
      $url = sprintf( "http://realcam.club:9710/http/send-message?username=admin&password=admin&to=%1$s&messagetype=sms.automatic&message=%2$s", $billing_phone, $message );
  
      try {
  
          // GET the response.
          $response = wp_remote_get( esc_url( $url ) );
  
          // If the response is a WP_Error object, jump to the catch clause.
          if ( is_wp_error( $response ) ) {
              throw new UnexpectedValueException( $response->get_error_message() );
          }
  
          // Log the successful response to the debug log.
          error_log( print_r( $response, true ) );
  
      } catch ( UnexpectedValueException $e ) {
  
          // Log the error to the debug log.
          error_log( $e );
  
      }
  
  
  }
  add_action( "woocommerce_order_status_processing", "mysite_woocommerce_order_status_processing" );
  

  and check what’s in the debug.log file in the wp-content directory of your WordPress installation.

This code will log the response or, if the request was unsuccessful, an error message to that file. I think it is straightforward enough for you to understand, but if you do have any questions or if anything goes wrong, please let me know.