It's not yet implemented but will be coming soon:
VOICE CHAT
We know that communication is key when you’re squadding up for that Victory Royale, so we’re working to bring voice chat to mobile. On top of that, you’ll be able to chat with your teammates regardless of platform!
We’re also looking to make it really easy to mute yourself, other players, or open up all communications with a simple tap. There will be a button on the screen that lets you mute yourself, mute everyone in your party, or go back to open-microphone.
Source: https://www.epicgames.com/fortnite/en-US/news/state-of-mobile
