I’m also on T-Mobile and on my iPhone, ‘WLAN Call’ is active when I’m at home. However, when I tried to see whether or not it was actually on, it said ‘T-Mobile NL’. I had to enable Airplane Mode (with WiFi still active), wait a few seconds, and then turn Airplane Mode off. Only then the carrier name switched to ‘WLAN Call’. Perhaps you could try this and share your experiences?