I know because it's working on my iPhone 6s. To check and see on your iPhone, go into Settings, Phone, and enable Wi-Fi Calling. Before today when I did that, I got a message saying that Wi-Fi Calling is not yet available.
The setup process is a few steps in length because you have to read and agree to the terms of service. And you need to enter your home address for 911-emergency call reasons. After I did that, my iPhone said the service would be activated in a few minutes.
I tested the service just seconds later however by disabling the LTE radio in my iPhone and then placing a call to my wife. She said it sounded better than a traditional call and that my caller ID came through just fine.