Hello prmaria.
Thank you for using Apple Support Communities. It sounds like you're experiencing some issues when using WhatsApp. Specifically, when you're on a video call and you receive a new incoming call, your display encounters an issue. If this is the case, we'd like to look into this issue with you.
First, we do recommend that you install any pending updates on your iPhone. Just in case, the following resources can help:
How to back up your iPhone, iPad, and iPod touch
Update your iPhone, iPad, or iPod touch
We also have this helpful resource you can review. You'll find recommended troubleshooting steps you can attempt with the WhatsApp application. Please check out the link below:
If an app unexpectedly quits, stops responding, or won't open
Be sure to follow all the steps listed within.
We hope this information is useful. Best of luck.