You can mute the sound while on a video call. Remember that muting the sound will not turn off the camera. For that simply place a regular phone call.
To mute the sound while on a video call, simply click on the microphone icon that you can see on the screen during the conversation. Clicking on it will activate and deactivate the microphone.
If you don't see this icon during the call, gently touch the screen to make all the different icon appear.