Have look at the 18185 service.
Go to FAQ`s, then click on:
Am I able to use 18185 if I'm with NTL, TalkTalk or Telewest?
answer: Yes! Once you have registered your telephone number and payment details you can start using Call18185 immediately. From non-BT landlines just dial 0808 1 703 703 , wait for the voiceprompt and enter the follow-on telephone number.
I assume that Virgin is similar to NTL.