I have a similar issue using PremiumSIM provider with 1&1 carrier in Germany. I am not expert at all, but while following VoLTE on the Xperia 10 III (mark 3) - filling the gaps - #33 by tn71 I noticed there is no modem config for 1&1. Nevertheless, I have also a second vodafone callya sim in the phone, and also that one does not run on VoLTE. Interesting enough, PremiumSIM changed my SIM a few years ago, and before that VoLTE was working. Please keep us posted if you find out something.