On the third try, I think I figured out the WiFi calling issue on unlocked devices (Galaxy S9). The key is to have the T-Mobile SIM card in the device before starting the setup for the first time. Doing this, the first popup I got said that the device will be updated with carrier specific settings. Problems when using a combo of T-Mobile and Samsung devices aren't exactly new. For example, in the past we highlighted the Samsung phone app force closing bug which you can read about here. If you are one of the users facing the "WiFi calling updated" notification issue on Samsung, then you have come to the right place.