Tomohiro Ohsumi/Getty Images Wi-Fi calling is a service that allows you to make phone calls over Wi-Fi, if you don't have a reliable cellular connection. Wi-Fi calling is built into nearly all modern phones, and usually comes at no extra cost. The sound quality on Wi-Fi calls will depend on the speed of your Wi-Fi connection. Visit Business Insider's Tech Reference library for more stories. You've probably heard a thing or two out there about Wi-Fi calling.