You can start a voice or video call from any IM with a pal.  Open an instant message window with someone and click on the 'phone' or 'camera' icon on the top right of the window to start your call.

You can terminate the call, start or stop your camera, and mute or unmute your microphone using the icons at the bottom of the window.

You can toggle between audio and video, and a text window by tapping on the 'chat bubble' near the top right of the screen.  Tap the '<' near the top left of the window to go back to your audio/video screen.

Click the 'swap' icon at the top right of the screen to switch between your front and rear camera.

When you are finished with your call tap on the 'handset' icon on the lower left of the screen.