You can do it by going to chat option. You need to just click on available chat person to whom you are going to do voice calling. Once you get the chat box, upper part of the chat box consists of following options.
1. Start a video call
2. Add voice or video chat
3. Add people to this chat
Now, you should go to second option which is visible as icon of phone. To have chat, you and your counter part should have microphone to have voice chat. If you face any difficulties, re-dial.