Of course, every carrier has its own coping mechanisms for service problems, typically activated at the point the customer complains.
If you say "I don't get good reception in my home," for instance, T-Mobile will inform you of your phone's Wi-Fi calling functionality (if your Android device has received the appropriate update), while the other carriers will likely offer you a femtocell device.
It was a reasonable idea, after all: by providing a reliable service inside the home, a cellphone could replace the stalwart landline, whose only technological advantage in 2007 was reliability and voice quality.
Built on the little-used UMA standard, the service shipped on a couple of specially Wi-Fi-equipped mid-range featurephones and was promptly forgotten.
Of course, having Wi-Fi calling, advertised or not, is a big leg up that T-Mobile has over other carriers.