It is a device capturing and controling all the signals, which are being created by the whole system, especially the made and received phone call signals. The device is mostly used for connecting regular phone system with the Internet and backwards.
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.