Your phone itself has to support your carrier for VoLTE.
All phones do 4G data on all carriers, but only some phones will work on VoLTE for a particular carrier. (Yes, I realise this sounds insane)
If the phone did not do VoLTE in Android, it will not do it in SF.
If you did not test it before reflashing to SF, you can flash it back to Android, or try someone else’s X10III if you know of one.
Was the X10III sold locally by local carriers? If it was, VoLTE has a better chance that it was supported.
Sadly here in NZ, Sony did not sell the X10III in NZ, Sony did not include any configuration for NZ, and they do not do VoLTE on any network.
In some countries, apparently, they do work on one carrier, but not another, so you might try sims from all the different carriers.
FYI
NetMon VoLTE shows volte connection status
CellMon can show you the bands in use