It is a custom implementation which makes use of the Opus and Speex codex. I used UE4’s voice implementation as the starting point and ended up replacing pretty much everything from the way the online interface works, how the voice engine [including both recording and playback] works, and even the fundamental way packet routing is handled.
All that said it is a proprietary system that took about 2 months to make and a decent amount of testing to get working well, so I really can’t provide all that much info about it 