Matrix is an open-source, federated software protocol that allows different computers and servers to send messages to each other. It's a lot like email but it has more features like VOIP and video calls.

Anyone can install Matrix to run their own server, and you can also use your friend's server (or anyone else's) instead of installing your own.

My server is https://odat.xyz and you can use the Riot client to connect to it and serve your messages and calls.

The official servers are @:

Protocol: https://matrix.org

Client: https://about.riot.im/

You could also use those instead.