Rebtel, the Swedish VoIP provider, allowed for unlimited, free app-to-app calling without the need for a Wifi connection. Usually, when calling someone internationally, you pay ridiculous rates or you use an app like WhatsApp. 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. Rebtel’s success hinges on two things. First, its app scans the available technology in a country to place calls over phone lines, low-speed networks or high-speed Internet;Second, Rebtel has hit on a marketing plan that puts it front-and-center in migrant communities: It pays migrants to sign people up. The Swedish company is focussed to immigrants in the U.S. seeking to stay in touch with their families.