AT&T has finally rolled out its native Wi-Fi calling feature on certain smartphones, after the Federal Communications Commission granted the wireless carrier’s waiver request for the feature.

Starting today, AT&T customers with the latest phones, the Apple iPhone 6S and 6S Plus, as well as those with last year’s models, the iPhone 6 and 6 Plus, can make and receive calls using a Wi-Fi network as long as the handsets are also running Apple’s latest mobile operating system, iOS 9.

Wi-Fi calling can be useful whenever users find themselves in an area with weak or no carrier coverage. By using a local Wi-Fi network, such as the one set up at your house or available in a public space like a cafe or library, you can still make calls without a cellular connection. And because the feature is baked into the devices themselves, you and the person you are calling won’t have to download a third-party app or service.

Out of the four major US carriers, AT&T is now the third to roll out Wi-Fi calling, following T-Mobile and Sprint. AT&T had hoped to launch the Wi-Fi calling feature last month when Apple first released iOS 9. Last week, the carrier accused the FCC of dragging its feet on approving the waiver it needed to offer the feature. The FCC, which denied the claim that it was slow to OK the request, granted the waiver Tuesday.

In addition to T-Mobile, Sprint and AT&T, there are a number of Wi-Fi only carriers that default to Wi-Fi first before connecting to a cellular network. These include Republic Wireless, Scratch Wireless, as well as Google’s nascent Project Fi, which launched in April and is compatible with the Google Nexus 6P, 5X and 6.