I’ve found some quirks with my Xperia 10 III. Whichever SIM is selected for mobile data usually registers using LTE and the other SIM with UMTS (and consequently can’t register for VoLTE). But occasionally after a time I notice both SIMs connected with LTE and the 1st SIM registered with VoLTE, the 2nd not registering with VoLTE. Once I have both SIMs using LTE it seems to stay like that until I make a change such as switching the SIM used for mobile data.
This looks like some kind of quirk or bug with Sailfish, but maybe has something to do with the phone itself?