Wifi calling and Numbersync are two different "things" that both use the WiFi calling feature. Your phone will not use Numbersync since it is itself already a phone, no need to act like one the way an iPad would act like a phone. So, Numbersync is not needed for Wifi calling from your phone. As for traveling and using the feature, I don't have personal experience. I assume it might work but then again it might be restricted. WiFi calling is not like Skype. WiFi calling is a replacement for the wireless connection to a cell tower. In areas where cell service is low and Wifi is available you have the option/capability to reach the mobile network via the internet. At a point, you reach the same call transport equipment that you would have reached using the wireless cell signal. Think of it like WiFi vs Ethernet at you home. Cable or wireless, you still hit your router to go out to the internet, its just two different ways to reach your router. So, traveling might still cost you roaming. I just don't know.