Fluxer is released under the GNU Affero General Public License. 🤷♂️
I'm going with Fluxer for the time being as that was the easiest setup as far as I could tell — on par with Discord. Just sign up, create a server, connect to voice. Blam-o. No payment necessary for what I'm using it for. Golden, simple, clean, focused UI.
I'm not sure which features you call basic which are not free of charge, but voice, chat, and screen sharing are all free last I checked.