Wifi Calling is a handy application that lets you make free calls over the internet, no matter what phone you use or what network you connect to. Just download the app, install it on your phone, and you're ready to go!

Wifi Calling supports all four major US carriers (T-Mobile, Sprint, AT&T and Verizon), as well as Republic Wireless and Google Project Fi. The latter two provide Wifi calling on certain phones too. Wifi Calling is also available on some iPhone models that run iOS 9.1 or higher. The app is free to download and use, and is a completely open source project.