Hi there, we're currently working on resolving voice chat issues but already have an open post for it. Please go to this post and vote on it so you can be notified of updates.
https://feedback.firstclasstrouble.com/report-bug/posts/voice-chat-dont-work

You need to be in game chat. And push L2 or the respective button on your machine. If it isn’t working. Check the right hand sign if the screen. If there’s small red writing, restart your game. Look at my bug report for more info