Whatsapp calls are actually calling the phone number

I have Verizon as Carrier. With my iPhone, sometimes I want to call my mother in Italy, on her Whatsapp account. So I open the Whatsapp app, go to the last chat thread with my mother and tap the top-right icon with the phone. Sometimes, though, the call is performed using my U.S. Verizon number rather than my WhatsApp account and therefore Verizon has already charged me a few times for international calls!! How can I make sure to perform the call using Whatsapp rather than the normal phone?