Erm it is.. On EE it's there by default along with IPv6 and other features. Even when I went up to 5G on a new contract I still have the option. I don't have a EE supplied phone I have a Oneplus 5T and it still gives me the options.
On a side note I did put in a VM sim and I also got the options on this phone. One which was on the Ultimate Bundle when I had it.
Virgin have quickly realised they pushed the button too early maybe! (link broken now) For the iPhones point though, looks like iOS 13.3.1 is to be released early this week and we may see a carrier update to get Wi-fi calling activated and as Enlli motioned this could potentially activate VoLTE!
Could be a good week being a Virgin Mobile customer?! (Maybe)