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.
Rates of the call shop are usually defined by the administrator, but the rate plans are managed in the main database of the billing system. All the rates are dependent on the charging policy; in case of a destination based billing rates are specified according to the area codes in the supported destinations.