Billing can be a complicated process for VoIP service providers and calling card operators - especially if they want to diversify and offer more than one service.
In some cases customers end up receiving separate bills or a third system has to be used to generate them.

A growing number of UK broadband users plan to start using VoIP in their homes, according to research.
The study by consultancy Booz and Company found that 44 per cent of consumers plan to use VoIP in the next six months.