This is what worked for me (it sucks, but I can’t figure out another way); NZ Adding VoLTE to X10III - #35 by tn71
Have you tried this: VoLTE on the Xperia 10 II - filling in the gaps - #112 by koza
Except with a different mbn location; maybe, like:
echo “mcfg_sw/generic/common/default/vl/global/mcfg_sw.mbn” > modem.conf