ST WiFi Calling is an Android app developed by SmarTone that helps you save roaming charges for calls to any Hong Kong numbers and for receiving calls from anywhere in the world. With ST WiFi Calling, you can make and receive calls even in places where mobile signal is weak or suffers from interference.

The app allows you to use your own mobile number, so your receiving party can see your usual caller ID. It also supports voicemail together with alerts for any missed calls and call waiting, letting you keep your usual connecting tone. Additionally, it shares your handset’s phone book for easy contact management and supports Dual SIM and eSIM. The user flow is simple: connect to WiFi, then open the "ST WiFi Calling" app and keep it running in the background to call out or receive calls.