Matrix is a modern, open source software for decentralized communication (like e-mail). Matrix offers end-to-end encryption, voice and video calls and much more. However, the most important thing is that there is not one Matrix server that all users have to use, like WhatsApp, Telegram and many other popular messengers.
Users can choose a public server or create their own and still communicate with all Matrix users, not just those on their own server. The messages are only stored on the participating servers. Calls also go only through the participating servers.
You can read the server rules here.
Feel free to create an account on my server or choose one from the list linked below. I look forward to hear from you @samuel-p:matrix.sp-codes.de.
On my blog (German) you will find a simple introduction to the Matrix network: Werde Teil der Matrix