OK. I think that’s why. When at home with my carrier (Orange France), VoLTE works fine, but as soon as I go abroad, I can’t connect to VoLTE, even though the same SIM with e.g. an iPhone works fine.
Must be a setup somewhere in SFOS to tell it “hey you’re roaming but you can use VoLTE on that network” 
This is a bit of a shot in the dark, but is the phone picking up the T-Mobile data access point correctly? (Settings → cellular network → data access point) It should be fast.t-mobile.com and protocol dual.
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