In case this ever comes up: I've had great luck using a T-Mobile sim in the ZTE Axon 7 Android smartphone.
This phone comes unlocked for "all 4 carriers", but in reality it works great with GSM-based carriers like T-Mobile and not so well with Sprint or Verizon.
I can get Band 12 (aka "Extended LTE") and make Wifi calls. I'm not sure about VolTE, but I don't know how to verify if it works or not and that is why I say I am not sure.
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