WhatsApp
https://www.whatsapp.com/WhatsApp is a fast, simple and reliable way to talk to anyone in the world. More than 1.5 billion people across 180+ countries use WhatsApp to stay in touch with friends and family, anytime and anywhere.
WhatsApp is not only free but also available on multiple mobile devices and in low connectivity areas — making it accessible and reliable wherever you are. It's a simple and secure way to share your favorite moments, send important information or catch up with a friend. WhatsApp helps people connect and share no matter where they are in the world.