Whether any phone has the capability to use VoLTE depends on whether the manufacturer has obtained certification for it with a specific mobile provider or providers. In other words, the device may have VoLTE with a certain carrier, but not with another.
Examples:
Sony XZ1 Compact (released 2017) is certified for VoLTE with T-mobile USA, but not with AT&T or Verizon.
Sony XZ1 (non-compact version, also released 2017) is not certified for VoLTE with any U.S. mobile provider.
So I think you’ll need to ask the mobile provider you intend to use, and specifically which model number is supported. (There are three different ones. And by the way, I would avoid the Japan-market model: SO-05K. I’m not sure if their bootloader can be unlocked, or if a custom ROM can be installed.)
EDIT: This may be helpful when choosing the model number that matches the most LTE frequency bands used by VodafoneNL, but understand that it may not take the VoLTE requirement into account.
It appears it gets better LTE coverage on other networks.