Here's the trick! I can't believe that this really worked. So, I have an iPhone 8 plus next to my 1 III. The Xperia is my daily driver of course. iPhone just carries my business SIM. So! As you expect, iPhone 8 is "supported" on VoLTE and it works too, both volte and vowifi, so I decided to give it a try and put that SIM in the Xperia. Et voilà, VoLTE and not just that, but the vowifi option popped up too and they worked flawlessly. So all you need is another phone that works on volte, put your sim card in it, enable and use it a bit then put it back in Xperia

My personal SIM is 5G and 4G enabled, but isn't volte by the carrier so it didn't work for that, today I'll contact them about it.