I purchased the Axon 7 (mini) because, as far as I know, it (full size and mini both) is the only phone that can have 2 SIM cards and be enabled for Wi-Fi calling on the T-mobile network, as long as it is updated to Android 7.1.1 (nougat). The Wi-Fi calling worked when I tried it in the USA. Then I went to the Caribbean West Indies (St. Lucia) and tried to turn on Wi-Fi calling there. It would not work. After many hours on the phone with both ZTE USA and T-mobile, I was able to determine that by accessing the notifications, where a red telephone/wi-fi icon showed up at the top of the home screen, an error code was displayed. With this information the T-mobile representative was able to somehow send information to my phone as I restarted it, and voila, Wi-Fi calling is enabled. The error message said there was no internet connection, however, there certainly was an internet connection. In the meantime, as advised by the ZTE representative, I had also gone through some complicated steps to make a new APN (Access Point Name), with the help of the T-mobile representative, who informed me that it was really not necessary. The place where you can change the APN is Settings/Dual SIM settings/scroll down to "Cellular Networks"/ Access Point Names/tap the + symbol.
The APN choices originally on the phone were "T-mobile US LTE", and "T-mobile US tmus". When I choose "T-mobile US LTE" the Wi-Fi calling now works. So it was not the APN settings that made a difference, it was seeing the error code and having the T-mobile representative reset something in the phone, so that Wi-Fi calling can recognize that there is an internet connection.
This took a lot of time, so just in case it helps someone else I am documenting my experience here.