Discord might be an alternative for team/friend based conversations. They have group calling (don't need to share a server) now. Problem is that it's marketed towards gamers and so might not be fit for purpose for business purposes
If your primary concern is open source then mumble/jabber is probably your solution
If you're truly paranoid there's a Mumble port for OpenBSD, though that obviously wouldn't fit Stallman's definition of Free.
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.
We tried Skype which is almost completely broken (used to work fine).
We tried multiple WebRTC solutions like appear.in which only work fine up to three people.
We tried Google hangouts, which worked fine but we didn't like the quality.
Mumble worked fine, but only if people use push-to-talk. Otherwise you get horrible noise. Every scratching goes over the line.
Skype Business supposedly works very well. We don't use it because we don't support Microsoft breaking the regular Skype.
We currently use the audio conferencing feature of (paid) slack. It works extremely well and has great noise reduction.