Viber

Viber is an application for iOS, Windows Phone and Android OS. It lets users make free phone calls and send free text messages to anyone who has the application installed. Viber works on both 3G and Wi-Fi networks. A version for BlackBerry devices is in the works, according to Viber. 
As of March 2017, this application is said to have 800 million registered users worldwide. In 2017, the app introduced a new feature "secret chats" wherein, the users are given an option to start conversations that self-destruct after a certain time limit.