Once that was all set up, a factory reset of the HTC 8X was performed. Once the phone rebooted, the blue Wi-Fi calling tile appeared on the home screen. The service now allows him to make free phone calls over Wi-Fi, the same way that the entry-level Nokia Lumia 521 does for its owners.
Let us know if this little trick worked for you. If you are an HTC 8X owner using the T-Mobile network, follow the steps above and let us know if you are now able to access Wi-Fi calling on your phone.