Ring 0345 600 0789 from any other phone. In some cases if you try ringing enough it will let you through to 789 as some people in the same situation reported.
Or use the Webchat from a laptop/desktop, between 8AM to 8PM M-F, 9AM to 5PM Sa: Mobile -> Mobile Billing and Payments -> Chat online. All agents busy? That's not a problem. Simply keep hitting the page reloading button every 1 minute and check again, do this continuously until you get through to an agent.
______ Any opinions expressed by myself are entirely my own and do not represent Virgin Media in any way.