Tinode

Tinode is a free, unlimited, flexible open source communication platform that’s been built mobile-first. Rich message formatting, video and voice calling, voice messages. One-on-one and group messaging. Publishing channels with unlimited number of read-only subscribers. Multiplatform: Android, iOS, desktop on Windows and Linux. Connect to Tinode service or setup your own! Completely open source: https://github.com/tinode/chat/