A few lines from the article:
Usually, when calling someone internationally, you pay ridiculous rates or you use an app like WhatsApp. Without a WiFi connection, apps like WhatsApp or FaceTime Audio require users to dip into their data plan to have the call.
With Rebel, the app uses a data (or wifi) connection to place an app-to-app call, and then routes that call through local phone lines. This uses up no data and doesn’t require wifi, letting users make international calls on the go without cutting into their data plan.
Read the article at techcrunch.com