During a video call, there might be two principal reasons why you could see a white screen in lieu of the other user face during the call:
Usually the reason for you to see a white screen is that the system is trying to get the data for the video transmitted by the other user. While it waits for the feed to arrive, the phone will show you a white screen. This will disappear as soon as the image can be shown. Depending on your connection and the one of the other user, this can take couple of seconds.
If one of the user has not given the access to the camera of the phone to the Instantgo app, the application is incapable of showing you a feed for that person. In order to resolve this issue, the concerned user should go on the settings of its phone, look for Instantgo and allow the app to access the camera of the phone.
Let's always remember that the quality of what you are seeing will always depend on the quality of your connection. Usually a wi-fi connection is recommended for video calls as they demand more bandwidth to operate properly.