To access the call menu, you can select the Call Support tab on the top.
You can hear and see the video of the other participant through the new window that opens up. On top of the video stream, you have the following features (as marked in the above image):
In case another user calls you, the application will show a notification message and play a sound signal. The calls can be answered from any screen that you are working on, even while you are in the middle of a task, to get expert advice.