I have the European LTE version of the shield and am using a data/voice compatible SIM that I tested in another Android phone before. Data on the stock ROM worked too.
1. I have installed the wx_un_mo 1.2 OTA:
The phone app was there, though when I try to call I loose network for a short moment and just get an error message.
2. I've downloaded the NVidia sources (for Android 5.0.1) and compiled them with wx_un_mo set as target version:
I've got the phone app again, but same thing as before happened. If I try to call, I loose my connection.
I think we need to modify the baseband to enable phone calling (and other LTE bands), but I have no Idea how that would be done.
Data connections did work in every case, though I could only test 3G as the pre-paid card that I use does not support LTE.