For one to one chats and the occasional group chat? XMPP is pretty awesome and has lots of good clients (Conversations on Android, Monal on iDevices, Dino, Gajim, etc. on Linux)
If you want something closer to Discord and host public communities or very large chatrooms, Matrix is very good. You can self-host it with Continuwuity, and there are lots of clients too (Cinny, FluffyChat, Element, Fractal, NeoChat, etc.)
If you just need voice chat, Mumble is simple and works well.
For one to one chats and the occasional group chat? XMPP is pretty awesome and has lots of good clients (Conversations on Android, Monal on iDevices, Dino, Gajim, etc. on Linux)
If you want something closer to Discord and host public communities or very large chatrooms, Matrix is very good. You can self-host it with Continuwuity, and there are lots of clients too (Cinny, FluffyChat, Element, Fractal, NeoChat, etc.)
If you just need voice chat, Mumble is simple and works well.