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, tap > Settings.
5. Tap Mobile data or Call, and then find and tap Wi-Fi Calling > Connection Preferences.
6. Adjust your Wi-Fi Calling preferences.

   Once enabled, you should see the WIFI CALL button in the phone dialer.

If after you've installed a T-Mobile SIM card and the SIM card changed prompt or the Wi-Fi Calling option in Settings doesn't appear, perform a factory data reset to switch to the correct SIM card check mechanism. Make sure to save or back up all your data before you do a factory data reset.

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.