You have only subscribe the data plan from your Cellular Network and start sending free SMS, Photos and Short Videos from WhatsApp application software. WhatsApp is a software like a messenger for using cross platform mobile messages and you do not need of any extra credit of sending short messages to your contacts as usually happen, you have to only connected your Internet connection through your mobile GPRS or other data connection while using it on your iPhone, BlackBerry, Android, Windows Phone and Nokia Mobile Phones.
