Mumble's requirements are fairly low, you could get by with a $3/month VPS or even a RasPi. Maintenance is down to keeping the OS updated, which is trivial on pretty much any VPS Linux install, as well as Raspbian.
If you're truly paranoid there's a Mumble port for OpenBSD, though that obviously wouldn't fit Stallman's definition of Free.
Last time I tried it, you had to do stuff like generate certificates and open ports and while it wasn't too much of a big deal and improved voice quality a lot, everyone wanted to stay on Skype.
Until we moved to Discord. Which has equally good sound quality, a fantastic chat and role system, and the ability for people to just jump into your voice call instead of having to be invited (which was what killed Skype for us, for any number of people over 3).
I'm part of a bunch of guilds for just hanging out and playing video games. But I also moderate two Discord guilds for programming, containing thousands of people.
Discord's pretty sweet, in a way that Skype and Mumble simply can't compare.
I used Discord for a while when I played Rust (post apocalyptic sandbox MMO) and was a member of a clan on there. It worked great, and the Windows client is fantastic. Basically, Slack+TeamSpeak.
Mumble server hosting is ridiculously cheap. No one has to run one unless they think they are going to discuss stuff that is so sensitive that it would be worth someone's time to subvert a hosting company and monitor the channel. ... and then you can pretty much pick the country you want to buy hosting in so as to make that unlikely.
If your primary concern is open source then mumble/jabber is probably your solution