WhatsApp calls on an iPhone and "data" and "minutes"
Usually carriers distinguish


minutes: for voice calls using the standard phone app, to either other phones or landlines (which can be confusing since FaceTime calls and audio use data and not voice minutes)
sms (which can be confusing since the the message app uses data for iMessage and SMS for MMS and SMS)
data: for all other internet traffic generated by any app that doesn’t hand off to SMS or the phone app. 


WhatsApp uses data for both messages and calls. If you are in a mobile network, any activity you do in that app counts towards your data usage. If you are logged in to a WiFi network, data will be send/received over WiFi.

Contact your carrier for details how you ar set up so you can reconcile your bill and plan with how data usage on iOS records usage.