https://www.tox.chat/about.html

https://teddit.net/r/projecttox/

I was around when the project first started. IIRC, when the Snowden leaks were still fresh a thread was started on 4Chan suggesting someone should develop a truly secure p2p voice, video, and IM application. From that thread several devs got to work making it happen.

It took about year before it was starting to become usable, but the clients were still super buggy and it wasn't ready to replace Skype. A lot of people were interested in Tox because it was supposed to replace Skype (whose usability and security degraded significantly after Microsoft bought them); Google Hangouts was not very good, XMPP, Mumble, and Ventrillo offered free calling, but they were a little more technical than Skype. There were a few other voice services, but there really weren't any other free video chat services back then, so Skype was the de facto standard to beat.

That's when Discord entered the scene; they catered to gamers (who were badly neglected by Skype) and they provided free high-quality voice and video calls better than anyone else. So, once there was an easy, free, and high-quality drop in replacement for Skype people forgot all about Tox. This was a couple years after the Snowden leaks, so by this point most people didn't care about the privacy aspect of Tox, they just wanted something that worked.

As far as I know Tox works, but is pretty buggy. When I was following it the devs seemed to get stuck for years trying to balance security and usability; invariably the more secure it became they had to tradeoff making it less convenient to use with lower quality video & audio. It is a legit project, but it looks like the development has slowed to a trickle and not much exciting has happened with it in several years.

EDIT: I forgot Jitsi is a thing, too. The last time I tried Tox it was too buggy to be usable, but I've used Jitsi a few times and it works great.