The original poster referred to VoLTE support “hopefully with major carriers”. I think it refers to the issue that many carriers do whitelisting of devices and Sony are out.
My Xperia 10iii does not use VoLTE with 3 operators I tested. @MeeGo-Junky found the only one operator in my country that whitelists Sony devices. (I am considering changing to them in a few months when my 24 months subscription ends.) And we’re talking about a major brand like Sony which presumably many tourists bring with them. For very small players like Jolla or Reeder it is guaranteed to be difficult to find operators that whitelist these devices.
Posting again the link for @mettska with the big table of providers that support VoLTE (or don’t) in each country: