Only found this out the other day when I tried to change tariff on my new Virgin Mobile account. Apparently if you are a Virgin Media customer you can get unlimited Virgin to Virgin minutes (well, 3000 minutes but that is a lot).
This is great if you signed up for a tariff in the last 28 days or are about to and are on Virgin Media for your phone, tv or broadband. Even if you don't have friends or family on Virgin, you never know who you will be ringing and this does not affect your other allowances.
I am now on 150 mins, 500 texts and unlimited data for £18 a month and now can ring my wife or daughter for free without using up my airtime allowance.