TeamSpeak and Ventrilo still exist, Mumble does too, and Tox.
Trade offs for features and functionality with all of those, but I'm fairly sure those do all run on both Windows and Linux, at least, the latter 2 are open source, and the first two, well, you run your own dedi voice server, so that's at least less bad than Discord.
Hell, back in like, 2009, me and a friend group just had an eternal Skype group call, used that for gaming voice comms and general chat.
EDIT: Also, apropo of nothing... how's that card game framework going?
This whole year I've been mostly doing PT, recovering from injuries, hard to do serious game dev with a torqued up wrist arm and shoulder, but I am finally starting to get decent function back...
... it might be fun to try to basically remake Caravan from New Vegas, you think that would be possible in your framework?