This was so frustratingly hard to find because Virgin Mobile UK do not support iPhones! Their staff even say it won’t work. It’s a sim card and a phone for crying out loud! *Rant over*
Hopefully this helps you set it up.
First you need to know your voice mailbox number, if you don’t have it dial
and follow the prompts to setup your mailbox. If already setup, still dial 222 and when you get to the end, you will receive a text with your number.
Then follow this http://www.fryfilm.com/infopages/iphonecodes.htm
E.g. to activate Forward if busy, dial
*67*[voice mailbox no]#