I am having the same issue except I have cancelled all virgin media services but need to transfer my phone number over to another network. I texted PAC dd/mm/yyyy to '65075' and received the following response.
Hi. We're sorry, but we can't help you switch providers right now. That's because we're already processing the request you made to cancel your Virgin Media services without switching. Something not sound right? Please ring us on 789 from your Virgin Mobile phone or 0345 454 1111 from any other phone. You'll need to do this ASAP if you want to keep this mobile number. Thanks!
I have never had a security question and cannot pass security when calling about virgin mobile nor do I have or am able to access a virgin mobile account as I was never prompted to set up one for the virgin mobile SIM included in the virgin broadband package with oomph I took out in 2020.