A free app for Android, by hoyojar.
Free Phone Calls – Free WiFi Calling is an application that provides you with the opportunity to make free calls over the Internet to any phone number worldwide. The service is completely free to use, and you will not have to pay for the calls. The app can be used on any Android device that has an Internet connection, and it works over the WiFi and 3G/4G data networks.
The service provides you with the option to make calls to any number in the world, and you can do it from your own device or from any other device, even if the number is not in your own country.