AT&T customers in areas with bad cellular service now have the option of making calls over Wi-Fi while in the U.S.

The carrier on Thursday rolled out Wi-Fi calling just a day after the FCC issued a limited waiver allowing it to implement the feature. AT&T Wi-Fi calling works on the iPhone 6s, 6s Plus, 6, and 6 Plus. To use it, your device must be running the latest version of Apple's mobile OS — iOS 9 — and you also need a wireless account set up for AT&T HD Voice and, of course, a Wi-Fi connection.

AT&T said this feature will be useful in areas with a bad cell signal, like homes or offices with "dense building construction" as well as elevators and basements.

To set it up, go to your phone's Settings menu, and enable Wi-Fi calling. During setup, you'll need to provide an address where you're most likely to use Wi-Fi calling for 911 emergency routing. AT&T said this information can help first responders locate you in the event that you someday need to place a 911 call and your location data is not available. You can update this address in the Settings menu at any time.

"Once set up, your phone will use Wi-Fi Calling automatically in places where you have limited or no cell signal, but you do have a Wi-Fi Internet connection," AT&T network operations and services President Bill Smith wrote in a blog post.

Apple officially enabled Wi-Fi calling in iOS 8, but it required carrier support to work. You don't need a separate app or different phone number to use it. You also won't be charged extra for calls made over Wi-Fi to U.S. numbers, and it doesn't matter if the person you're calling also has the feature.

For more on AT&T Wi-Fi Calling, check out the company's FAQs and the video below.

Sprint has offered Wi-Fi calling since 2014 and expanded it to the iPhone earlier this year. T-Mobile, meanwhile, has offered it since 2007 and brought it to the iPhone last year. Verizon has tentative plans to introduce it sometime this year.