Another alternative is Revolt. It's open source and very similar to Discord, but it's not federated, and thus not part of the fediverse.

Matrix is okay, but not a drop-in replacement for discord. No calling.

I use matrix exclusively since a month ago. Works pretty much the same, not ad polished but you can bridge nearly anything to matrix. Whatsapp, Discord, Telegram, Signal etc. i highly suggest you check it out.

Matrix is great but in very much Work In Progress state. Many things are rewritten or not complete because of the low budget Matrix foundation has for all their ambitions.

I use it everyday, mostly without problems, but sometimes there is an error and my app cannot decrypt a message, sometimes calls do not ring... It seems like the best base for universal chat, but I don't know if showing it to my friends and family instead of waiting a year or two more was a good choice.

I’ve been using Matrix for the last few months, via Element which supports some newer feature like video and voice calls, and it’s been great. My only complaint is that their Video Rooms feature is kinda flaky with video, which is weird given that one-on-one video calls are rock-solid. Voice-only in a Video Room is reliable tho. Video Rooms are still an experimental feature that you have to enable in your settings.

I’ve seen a couple of Discord servers that use a Matrix Bridge to sync up Discord chats with Matrix Spaces. The only downside to that is that bridges and bots don’t support E2EE, which if you’re bridging to Discord, isn’t much of a downside anyway.