Mocha is OTT (over the top) product. User can send message by phone that has internet connection. I worked on this project as Android developer.
We are using XMPP protocol for interacting between client and server. I decided to use asmack (XMPP library for Android) and modified this library for our project. Moreover, I researched about VOIP technique in this project.
Currently Mocha has 2M download in Viet Nam.
You can download it in this link: https://play.google.com/store/apps/details?id=com.viettel.mocha.app