Wi-Fi is now an integral part of many wireless carriers' networks, and Wi-Fi calling is becoming more ubiquitous as the processes mature and handovers improve. While Apple (NASDAQ: AAPL) released its Wi-Fi calling feature last year, its next iPhone release, expected in September, will come with its Continuity feature to support Wi-Fi calling on Macs or iPads even when the devices are on different Wi-Fi networks. In fact, AT&T appears poised to join Sprint and T-Mobile US in supporting Apple's Wi-Fi calling service. While it might sound counter-intuitive, operators in general are interested in offering Wi-Fi calling because it's cheaper for their customers, especially when you consider the cost of international roaming. It helps carriers provide better coverage, and they don't have to spend thousands of dollars on new base stations. Plus, it's a harbinger of 5G, where all the connections will be treated as part of one big network. For more on the state of Wi-Fi calling and where it's going, check out this FierceWirelessTech special report.