Hello,
I left a message at https://wp-sms-pro.com/support/ (a screenshot left there)
This line in class-wpsms-pro-gateway-smsru.php may not work.
wp_remote_post( $this->wsdl_link . "sms/send", $args );
And sms.ru provides API key to authenticate 3rd modules, it should to be used to authenticate the plugin beside the username API and password API.
Thank you!
