T-Mobile only supports WiFi calling which may be similar. However, most unlocked devices will not have access to this feature.
To use Wi-Fi Calling, you need a phone that supports T-Mobile Wi-Fi Calling:
- Wi-Fi Calling is included on most T-Mobile devices. Check the the Devices page to look up yours.
- iPhone 5c, 5s, 6, 6 Plus and newer on iOS 8 or later have Wi-Fi Calling, even if not purchased from T-Mobile.
- Some manufacturers state their devices have T-Mobile Wi-Fi Calling (for example, Nexus 5x, Nexus 6p, Pixel 2).
- Most of these devices work on T-Mobile Wi-Fi Calling without any problems.
- T-Mobile does not test them for compatibility, so we cannot guarantee their service.
- Not available on iPhone 5 or older devices, even with iOS 8 or later.
This is in no way an answer to my question.