Have you tried this: VoLTE on the Xperia 10 II - filling in the gaps - #112 by koza
Except with a different mbn location that suits your provider; maybe or maybe a general one, like:
echo “mcfg_sw/generic/common/default/vl/global/mcfg_sw.mbn” > modem.conf
Was VoLTE working prior to installing sailfish?