Best answer by Gregor
Unfortunately there is nothing that iD Mobile can do for you. Your handset support WiFi Calling but iD cannot guarantee it to work if you have not purchased your handset with them then you might be running different version of firmware that iD does not support. When I had Xperia XZ1 Compact I had to flash different firmwares until I found one that worked with iD. Unfortunately it is kind of hit or miss situation here, unless you buy with them directly.
Here I have explained it in a little bit more detail.