I expect to be purchasing an unlocked Note9 directly from Samsung. My understanding is T-Mobile's Wi-Fi Calling will work on a factory unlocked phone, since this will notbe an unlocked phone purchased from another carrier. Thanks in advance!
It probably not come with support for the out of the box and not an easy button to enable it. It is possible to manually flash T-Mobile software on it if it is the same variant that T-Mobile sells. But that is not something that is supported by TMobile or Samsung so they would not be able to help you with that process.