WhatsApp comes with a useful built-in feature to help users reduce their data consumption during calls. If you'd like to take advantage of this new feature, you'll need to enable it first in the app settings.
Open WhatsApp, tap on the More menu > Settings. Tap on Chats and Calls:
Scroll to Call Settings and tick the Low Data usage - Lower the amount of data used during a WhatsApp call checkbox:
N.B. Data consumption will be reduced, but it should be noted that you are bound to experience minor loss in call quality as long when this feature is enabled.