Welcome to Vodafone Community
Posting this here as I hope it will help other poor souls with an XZ1 (and it will probably work with other Sony mobiles). There are other older posts on the forum on this subject, but sad tales.
So you have been on vodafone's site, followed the instructions and there is no option showing for wifi calling:
DO NOT try and use Vodafone's tech support (for reasons that will become apparent - if Vodafone had a strapline it should be "We make simple things difficult"):
Think you need to be running Android 8, My XZ1 is Android 9. Make sure your phone is up to date before you start.
My experience was very different, I'd bought an XZ1 compact (like Sony phones, must have wifi calling - they are turning suresignal off in 2021, my XA1 is not wifi calling capable). The seller put a vodafone SIM in it and demonstrated it working with wifi calling - given my experience, he must have spent an hour phoning/webchat with vodafone to find out how. He gave me explicit instructions on what to do. Not knowing the above - that I could do it myself, I contacted vodafone, telling them upfront what I wanted. Three people and one hour later they still hadn't done it and their 'expert' was telling me that my XZ1 was not compatible because I didn't buy it from vodafone. This in spite of me saying at least twice that I had seen it working with wifi calling on vodafone. They only disabled/enabled wifi calling when I insisted. And I didn't spot the notification until the following day. A lot of pain and grief. This is now subject to a formal complaint.