How do I silence incoming voice calls on WhatsApp?
In order to silence the ringer during an incoming WhatsApp call, you can turn off all the "Notifications," and prevent the WhatsApp from accessing your microphone by going to Settings > WhatsApp >Notifications. Please note that this will disable all the notifications on WhatsApp including the chat/message notification.   

 

If you'd like to receive sound notification for text/chat and only disable the ringer for incoming WhatsApp call, then you should disable the "Banner" alerts, and prevent the WhatsApp from accessing your microphone by going to Settings > WhatsApp > Notifications.