While the WhatsApp app itself is free to use, you may be charged for a call if you are using cellular data instead of Wi-Fi, especially if you are roaming internationally. Unlike a traditional phone call that uses cellular minutes, a WhatsApp call is a "Voice over IP" (VoIP) service that "consumes" data. An audio call can use about 0.5 MB per minute, while a video call can use significantly more. If you are abroad without an international data plan, your home carrier will charge you exorbitant "roaming" rates for every megabyte of data used by the app. Another common "surprise" charge occurs when your Wi-Fi signal is weak and your phone's "Wi-Fi Assist" feature automatically switches to cellular data to maintain the call quality. To avoid these charges, always ensure you are on a stable Wi-Fi connection and turn off "Data Roaming" in your phone's settings before making or receiving calls while traveling.