What's the difference between VoIP and Wi-Fi Calling?

VoIP calling uses a third-party application (like WhatsApp or Skype) to make calls over any internet connection. 'Wi-Fi Calling' is a specific feature built into your phone by your home carrier that allows you to make and receive standard cellular calls over a Wi-Fi network when your cell signal is weak. Crucially, using your carrier's Wi-Fi Calling feature from abroad may still be billed as an expensive international call. VoIP app-to-app calls are always free.