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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I have the same Issue. I’m using Swisscom as my provider. VoLTE works great while I am in my homenet (Swisscom, 228-01). Device is a Xperia 10 III with officially licensed SFOS 4.6.0.13.
But when I am roaming in Germany or Austria, VoLTE cannot register, regardless which network I am logged in. At least in Germany, neither Telekom, Vodafone nor O2 let my register. The button in the settings just blinks forever and calls drop to 2G.
If you are interested you may check the GSMA resources covering VoLTE roaming issues:
https://www.gsma.com/get-involved/working-groups/key-topics/volte-roaming
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