Xmpp clients are completely awful on Linux desktop, and can't do voice and video chat correctly with clients on Android like Conversations
There is not a single good XMPP client for macOS or iOS either. There are a couple of minimally functional ones that are aesthetically dreadful, but nothing that would come even close to replicating what people would expect even from Signal or WhatsApp, let alone a monster like Discord. Certainly nothing that I could ask friends or family to use.
You can with dino. I wish I had the time to work on getting A/V calls back in gajim. It could be worth asking an NLNet grant for that…
https://dino.im/