Hi. This option is available and does uses your sim card balance. If there isn't any signals from your mobile network and your mobile is connected to WiFi than you can make or receive calls on your sim number. Remember you can only make/receive calls. Sms isn't supported
@andleeb786, when the gov shuts down mobile network and you are in home using wifi, same time you can receive or make calls using ur sim card as long as you ahve good working Internet connection
