Unfortunately, Vodafone only enables wifi calling on devices it has tested and these tend to be ones that it's sold (although you don't have to buy from the network for it to work). You can see the list and find implementation guides here, but I'm afraid the Z5 isn't on it.
Quite a wide range of Sony devices is included, so it's possible that yours will get the facility later and the relevant toggle should appear under Settings|Network. It may even enable by default (my wife's SIM-free L3 worked out of the box).
Approval by model is slightly bizarre, as it's probable that (apart from different versions of Android), most devices across a manufacturer work in very similar ways. I have a Moto G8, which isn't enabled, yet one of the G9s is - since Motorola use virtually pure Android, it's very unlikely there are any, let alone significant, model differences.
In the meantime, I'm afraid, all you can do is wait. Sorry.
