I was with O2 for about 14 years before deciding to switch to Virgin, as Virgin seem to offer a better coverage due to being on the EE network (I can now get 3G or 4G almost everywhere I go - with O2 it was patchy), are much cheaper, flexible and offer better customer support.
However, I live in a little rural village and while I get a great signal outside of my house, I usually get no service or a very poor signal within my house. O2 had a calls-through-wifi app, which was handy. I notice that if you are directly with EE you can simply switch wifi calls on in the iPhone settings.
Does Virgin offer anything like this or have any plans to?