While Sony's recently announced smartphones will not be making their way to any of the wireless carriers in the US - in fact there will not even be CDMA models, so that Verizon and Sprint users are completely out of luck - Sony did make sure that these smartphones work with T-Mobile's network technology. So instead of just working on its network with the right bands, Sony went through the certification process to get WiFi Calling, HD Voice and VoLTE supported on the Xperia XZ1 and Xperia XZ1 Compact out of the box.
For Sony's new smartphones to have this capability, it's actually a bit surprising. Typically only smartphones released on carriers are able to support this technology for T-Mobile or another carrier. But now users will be able to pick up either the Xperia XZ1 or Xperia XZ1 Compact and use it on T-Mobile with WiFi Calling, HD Voice and VoLTE without any of T-Mobile's extra apps installed, or even T-Mobile's logo on the device. That's a win/win for customers. These are both GSM-compatible smartphones, so they will both work on AT&T as well as other GSM carriers outside of the US. They just won't be compatible with Sprint and Verizon, unfortunately, but that is something that was inevitable really.
The Sony Xperia XZ1 and Xperia XZ1 Compact are essentially the same phone, aside from the screen. The Xperia XZ1 has a 5.2-inch 1080p display while the Xperia XZ1 Compact has a 4.6-inch 720p display. Both are powered by the Snapdragon 835 and 4GB of RAM. The Xperia XZ1 Compact has slightly less storage at 32GB while the Xperia XZ1 has 64GB - both have micro SD card slots for expanding storage if needed. Both also have that new 19-megapixel camera around the back, which is capable of doing 3D scanning, which is a rather interesting and under-baked feature at this point. But it should become a rather interesting one in the coming years. Both smartphones will be available unlocked in the very near future, Sony has not yet announced pricing or availability, other than the fact that it "should be available in September".