And just to confirm Enllis comments.
A few weeks ago I was on the Ooomph package which meant, as well as the Broadband and TV bundle I got a 'free' SIM that was labelled Virgin but piggy backed the Vodafone network (to further confuse matters a few months ago the same Ooomph bundle provided a Virgin labelled SIM which piggy backed the (far far better) EE network.
Anyway if you move to Virgin Volt you get a new SIM that is from O2 direct and your Virgin SIM is deactivated.
I suppose the confusion is that with Virgin/Vodafone you got one bill from Virgin for all services (including mobile). Now it is split into two - one for the Virgin services of broadband/TV (and Netflix if you are lucky lol) and one for O2 which is managed directly by O2 and not Virgin (as Vodafone was).
Nobody said it was easy LOL