Wi-Fi calling now available for AT&T's Apple iPhone 6/6 Plus and Apple iPhone 6s/6s Plus
Yesterday, we told you that the FCC had finally granted the requested waiver to AT&T, allowing it to begin Wi-Fi calling service. And today, the nation's second largest carrier announced that Wi-Fi calling is now available for the Apple iPhone 6, Apple iPhone 6 Plus, Apple iPhone 6s and Apple iPhone 6s Plus, as long as they are running on iOS 9.
To use Wi-Fi calling on your eligible iPhone, go to Settings > Phone > Wi-Fi Calling > Wi-Fi Calling on This Phone (Toggle on). Once the service is enabled, it will allow you to use make calls over a Wi-Fi connection, even in a weak cell area.
source: CNET (1), (2)