Long answer: Some people have suggested Virgin may be looking to launch a WiFi calling solution in the medium/long-term, perhaps once the new "full-MVNO" purple SIMs have completed the rollout. Speculatively, I wouldn't be too surprised, but VoLTE could still be a problem. EE will ultimately want to increase the proportion of users on VoLTE as opposed to CSFB solutions, but equally will prioritise their own PAYG customers first. And for as long as their network can support enough non-VoLTE users will probably be quite happy to keep it as a point of differentiation for themselves.
Ultimately the people within EE & especially Virgin who "know" won't be posting too much on these forums!!