update: I'm running G8343 (Canada).
Pulled G8342 (US) and replaced the 3 sin files, deleted userdata.sin and all *.ta and ran newflasher. I ended up getting the "no sim found" message even though everything flashed fine to the G8342 US version. Does this happen to G8341 Int'l folks? (is it a single sim conflict?)
Took the original G8343 firmware and just replaced the oem_X-FLASH-CUST-C93B.sin from the XZ1C (US). It loaded up, but no VoLTE option. After letting the phone run on T-mobile, it asked for a reboot to optimize the cell connection. After reboot, VoLTE option showed up.
Problem now is cellmapper says I have Band 12 tower down the street, but it WILL not connect to band 12, only Band 2/4. Tmo's BYOD app says no too. I confirmed VoLTE and wifi calling are provisioned (*#*#4636#*#*), but any ideas why it would not connect to Band 12? Sim card/imei needs to be registered (it's ~2 yrs old)?