Yes, it is absolutely free of charge.
No, only the international numbers who called you while you were away will receive the Notify Me SMS.
Yes, you can manage the service as you like.
No, you can activate and deactivate the service at any time for free.
Yes, the service will continue to work normally and no charges will apply to you.
Yes. However, the USSD code could be blocked by some of the operators or used for their local services.