This is partially true ...
CC has their own network infrastructure but a CC subscriber will "roam" on VC network in areas where CC have bad coverage ...
The thing to bear in mind is that VC will give preferentially service to their own subscribers especially in high-usage areas.
e.g. A call by a CC subscriber roaming on VC is more likely to have their call dropped than a native VC subscriber. Given that Virgin is just another CC number range ... one can only imagine the implications.
