Why can't I use T-Mobile's Wi-Fi Calling feature on my unlocked HTC phone?

If your phone is the unlocked edition, it supports the T-Mobile Wi-Fi Calling feature. If you don't see the Wi-Fi Calling option in Settings, you may need to enable it while you're on the T-Mobile network.

To do this:

1. Turn your HTC phone off, and then insert a T-Mobile SIM card to the phone. See the user guide or how-tos on how to insert the SIM card.
2. Turn your HTC phone on.
3. If you see the SIM card changed prompt, tap OK to restart your phone.
   Note: The prompt may not appear if you've inserted the T-Mobile SIM card when your phone is powering on.
4. After your phone restarts, connect it to a Wi‍-Fi® network.
5. From the Home screen, swipe up and then find and tap Settings.
6. Tap Call > Wifi Calling or Call > Wifi Calling > Connection Preferences and select your Wi-Fi Calling preference.
7. If Wi-Fi Calling is not enabled yet, tap its On/Off switch.

   When you see the Wi-Fi Calling icon on the status bar, your phone is now ready to place Wi‍-Fi calls. Open the Phone app and you should see the WI-FI CALL button in the dialer.

   Note: Certain T-Mobile SIM cards have similar settings with other carriers. Slide open the notifications panel to check if there's a notification that prompts you to choose a carrier. Tap the notification, and then select T-Mobile as your carrier to ensure the Wi-Fi Calling settings will appear.