VoLTE on the Xperia 10 II - filling in the gaps

how does that apply to 4G/VoLTE when roaming?

I have a X10ii and live in France with Orange as provider - VoLTE works fine at home.
I’m currently in the US, therefore roaming - T-Mobile is the provider I am on.

When doing steps 1 and 2, I see that my provider is still Orange, as I’m getting S11.3 and the modem.conf file contains:

mcfg_sw/generic/euro/orange/vlvw/fr/mcfg_sw.mbn

I tried using the workaround suggested here but unsurprisingly, since the value for me is S11.3 and it already exists, that did nothing.

So I’m not quite sure if I’m supposed to do something? Or is roaming just not taken into account at the moment?