How do I use voice call during a game?

I noticed there's a phone icon during chess matches. I asked a user if he wanted to voice chat.
We pressed the button, and it just blinked and said, connecting. How can we use it?

Even i am wondering. It never works for me. Maybe a mic is needed.

It works well on Chrome browser.Can help you test if you wish to. I have mostly used it for Study / Game Analysis.

When you press it your opponent can hear what you say

When I press nothing happen

You and your opponent have to click the phone icon!