AT&T offered Wi-Fi calling in iOS 9 beta, but removed the feature as it did not comply with FCC's regulations that mandated teletypewriter support (TTY) for the hearing-impaired. The carrier has obtained a waiver that allows it to roll out Wi-Fi calling using a newer form of accessibility service called real-time text (RTT).
While announcing the news, AT&T also challenged T-Mobile and Sprint, stating that they did not seek out a waiver for lack of TTY support in their implementation of Wi-Fi calling:
We're grateful the FCC has granted AT&T's waiver request so we can begin providing Wi-Fi calling. At the same time we are left scratching our heads as to why the FCC still seems intent on excusing the behavior of T-Mobile and Sprint, who have been offering these services without a waiver for quite some time. Instead of initiating enforcement action against them, or at least opening an investigation, the agency has effectively invited them to now apply for similar waivers and implied that their prior flaunting of FCC rules will be ignored. This is exactly what we meant when our letter spoke of concerns about asymmetric regulation.
We'll let you know once AT&T announces a specific date for the rollout.