It is the business practice of sending all clients' billing info to a separate company, who then does everything for you. It also allows a company to cut down on time and human resources spent on administrative tasks.
Because Google Voice provides such a competitive and feature-rich service that not only includes free VoIP and SIP calls across North America and internationally at fantastic rates, in addition to a huge range of services such as voicemail, conference calls and voice transcription, it was inevitable that since its inception it would attract the ire of other competitors who would block it, or sue in courts to have the government legislate against it.