Make sure that HTC Sensation 4G is connected to a Wi-Fi network before you turn on Wi-Fi Calling.
On the Home screen, tap
Read the important notice about emergency calling, and then tap Dismiss.
Tap the Wi-Fi Calling switch.
HTC Sensation 4G then connects to the T-Mobile network and shows Active on the screen when connected.
You can then go to the Phone dialer, Call History, or Contacts app to make calls.