Sim boxes are a system which contain sim cards of O2,Orange,T-Mobile,Vodafone and Hutchison 3G(3 Mobile).
What happens then is you dial a mobile phone number, an O2 mobile phone for example.
The call then gets routed through an O2 sim and that probably costs Virgin Media 5p per minute, O2 dont mind because thats how much they would get if the call was routed in the normal way. If your not on Talk anywhere or Talk Mobile then you will be charged around 15p per minute plus 7p to connect. So both Virgin Media and O2 are happy bunnies cashing in!
When we hear those recordings its probably because the mobile phone companies have not set up the sim cards properly or that Virgin Media have not paid enough funds towards them. Or simply because Virgin media have requested disconnection and the mobile phone companies have converted them to pay as you go sims.
Also, In the 1990's when Cable companies started launching there own phone service, it wasn't really there own service, they were just doing the dirty work for Mercury (Now called Cable and Wireless). In other words, the cable companies installed phonelines right from the cable and wireless infrastructure to peoples homes, Mercury's network didn't reach many residential areas. It wasnt a success as number portability was not possible back then. All calls except local calls and maybe some calls to national BT lines were routed through Mercury communications (Cable and Wireless).
Now that NTL/Virgin Media has taken over all except 2 cable franchises, it has made it easier to build a national infrastructure and means there is no more middle men to waste money on, meaning that a call from an Virgin phone in Manchester can get through to a Virgin phone in London without being routed through another phone company saving Virgin Media a lot of money!
And lastly, most of the time companies do not pass the above savings on to customers because there is a lot of maintainence and upgrade work and some profit to be made too!