Wi-fi Calling not in Settings > Phone after AT&T rollout

AT&T has rolled out Wi-Fi calling. It's official. I live in a major metropolitan area where it is available. My iPhone 5S is a supported device.

So far, every article I've read on setting it up says go to Settings > Phone and enable it. If you get an error message, it's not available yet in your area.

That means the setting has been visible to phones that don't have the feature yet.

I, however, don't even have the setting to switch on. Why?

Why would do some supported phones display the feature and some not? Is it a carrier update? Is it because I ran the iOS 9 beta? (I'm now off the beta with iOS 9.0.2.) Do I need to call AT&T to set something up? AT&T's website is no help-- neither the fancy press release nor the FAQ acknowledge this scenario.