WiFi Calling only works when using T-Mobile service. Using an overseas provider's SIM will not allow. Using a T-Mobile SIM will allow WiFi Calling, just like if you were in the US, since your SIM would be a US number. IOW, the SIM is connected to T-Mobile. If you use another carrier SIM, it isn't T-Mobile.
What a sticker shock!
This is then just a 'T-Mobile-Only WiFi Calling' feature.
Not really a general/Android phone feature anymore. The phone's 'WiFi Calling' should be labeled as such accordingly,
So many people have been misled by thinking WiFi calling feature on the phone should work elsewhere as well.
Any other phone features on this phone only work with T-Mobile account/network only? (not talking about frequency bands of course)
For people who are travelling, this is a complete turn-off in buying phones from T-Mobile for more than local account/network usage.
Obviously, as a consumer who travels I'd want to get a phone with features that are not tied to the specific locale network. Particularly features like WiFi calling that helps so much when inside buildings/cell signals are weak.
I thought what a nice feature T-Mobile phones had. That was one main reason I bought T-Mobile phone vs. regular unlocked phones.
Now there's a LOT LESS incentive to buy T-Mobile phone, I'm seriously looking for phones outside T-Mobile that would have WiFi calling feature, or persuade Google Android OS to have built-in WiFi calling on its own independent of specific carrier account.