I'm visiting Hong Kong now. I didn't sign up any Tmobile International roaming plan. First thing I noticed at the hotel WIFI connection, WIFI calling kicks in. Then I received calls from the US as well as calling back to the US just like domestic call. Awesome! However, when there is no WIFI presents, the phone is dead. I called Tmobile to unlock the Sensation. Within 24 hours, I received an email with the unlock code and the instructions. Bought a prepaid SIM card at CSL/New World. It includes certain talk minutes. It also have daily or weekly unlimited 3G data. The unlimited 3G weekly rate is HK $78 which is just about US $10. Loaded the card in, I was expecting to see 2G connection. To my surprise, the 4G icon shows up. I'm pretty sure it's 3G. I didn't test the speed but all the apps loads very quickly. Then, whatsapp and viber continues to work even a different sim card is in. Very happy.