That's super bummer to hear this and we really want you to be able to use the service. In order to use Wi-Fi Calling you'll need a device with that installed. This does come with all T-Mobile branded device, but those that aren't may not get this capabilities.As for the coverage at your home, we'd need to get a Service Complaint filed in order to help get some coverage improvements for that area.
The Sony XZ2 does support wifi calling and can be seen in the hidden menu for the phones network connections (but it is greyed out with tmobile). The issue is that T-Mobile doesn't allow the device to use wifi calling because it is not a certified device. This is 100% a tmobile decision and should be corrected. Here is a link showing how to enable wifi calling for the XZ2 How can I enable Wi-Fi Calling (VoWiFi)? – Sony Xperia XZ2 support (English)