This isn’t really an answer, but I spent a lot of time trying different voice chat systems out a couple years back, but couldn’t get my social group to move off Discord.

So, the real answer is any that you can convince the people you want to talk with to use. However other options include Mattermost, Jitsi, Mumble, Teamspeak, and Nextcloud Talk off the top of my head. I think I remember seeing that rocket.chat has voice now too.

Mumble or Teamspeak is my vote as well. Both solid voice chat systems that work extremely well. Teamspeak is more user friendly and easier to use.

Both are real group voice chats as well, not the “meeting room” group chat that’s a pain to use like Jitsi/Zoom and similar.