WhatsApp uses your phone's Internet connection (4G/3G/2G/EDGE or Wi-Fi, as available) to send and receive messages to your friends and family. You do not have to pay for every message. As long as you have not exceeded your data limit or you are connected to a free Wi-Fi network, your carrier should not charge you extra for messaging over WhatsApp.
WhatsApp Calling works in the same way. To learn more about making calls over WhatsApp, read this.
Please be aware that:
WhatsApp Support Team