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.
http://appear.in/some_words_doesnt_matter <- share with whoever, instant video call
But, the boss will see when you're "available" straight from Outlook, which is where he lives. It's not an IM tool, it's (yet another) instrument of workforce control.
Also, despite the rigid security policy where I work, the mac client stores my password in the keychain.
I don't think it's likely to happen any time soon unfortunately.
Regarding Facebook, it still feels weird, I'm not sure I can put my finger on exactly why. I just want to keep "friends" and "business" totally separate.
Once you invest more in working around this by having native clients, it makes sense for your product to do more.
There are a lot of games and software communities that use Discord quite productively, significantly better than what Mumble or Teamspeak have ever allowed for. Just having actual textual channels has made it so dev talk about modding, server administration and all things technical is just a few clicks away. Discords devs do impliment features that are useful for other developers, such as code markdown support. And unlike Skype, you can actually talk to the devs in the official Discord Devs server if you catch them. I'm not an important person at all, and have talked to the CEO and half the teams devs casually.
You can do: ```javascript var x = "y"; ```
and it'l syntax highlight. There is a clear roles permissions system, with support for multiple channels both textual and voice, full API support for building bots, and Discord uses their own service for development communication.
Of course its not a one-size fits all, Skype is more business oriented, so it looks more proffessional to use, but Discord is well on its way to be an actually better alternative from a technological standpoint. That sweet P2P video and audio will be great, and they've stated in the past they actually can't host that stuff through their servers, because even just proxying that much video data is way too costly.
Not to say its all perfect, some of the design decisions, such as making it so normal user accounts have the same access to the API as bots do (Though you'l be banned it 'caught'), makes it so users can easily just run a chat spammer. Its a fundamental issue because the program itself works in that manner. They also hate modding and plugin support, with some of the less experienced devs creating tirades against Javascript injection software like BetterDiscord, which allows you to add in custom CSS, and make your own javascript plugins to manipulate the DOM. Even though its incredibly useful, even basic things like changing font size of the program are only doable through that. They very much so have an Apple-esque idea that they know better, but it seems they've made a compromise (so far) that they haven't broken injection abilities yet.
slack probably should be able to create a full replacement for lync, (skype for businnes).
I used to think that, too, but with the rise of cloud offerings like Office 365, many orgs are starting to see that running your own servers for Sharepoint/Exchange/Unified Communication isn't worth it anymore.
They've deleted Skype numbers from my account without warning. Someone linked my and my wife's account because we used them on the same PC, and refuse to unlink Skype and Microsoft accounts.
I'd love to find an alternative, but given that everybody already has Skype accounts the lock-in is pretty significant.
Don't much like the alternatives Stallman proposes. Using long distance phone calls seems worse than Skype, phone companies have provided governments with even more access to user data than most tech companies.