U Mobile recently launched their WiFi Calling services for subscribers of their post paid and prepaid plans here in Malaysia for free, no additional charges. It turns U Mobile Network towers across Malaysia into WiFi stations allowing you to get HD Voice calls over weak mobile connections as long as you are connected to a WiFi with Internet, even your local mamak's.
This will allow you to make HD VoIP (Voice over Internet Protocol) calls through your existing mobile number without having to access an app like Skype or Whatsapp. For those U Mobile customers abroad, using WiFi Calling will not incur roaming charges and you can even make calls with in-flight WiFi connections.
Unfortunately, this can only be done if your device supports WiFi Calling. These include the Apple iPhone 5C, 5S, SE, 6, 6 Plus, 6s and 6s Plus that have been updated to iOS 9.3 or higher. To do WiFi Calling on your iPhone enable it at Settings > Phone > WiFi Calling. Android phones will be able to us U Mobile's WiFi Calling service in Q3 2016.
Check out the U Mobile site for more details.