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.
This function available in some telephone networks allows redirecting the incoming call to another telephone, for example, a mobile phone, a box of voicemail or another telephone number where the called subscriber is situated. In such a way the subscribers can, for instance, receive calls that have arrived at their work number from their home or mobile phone.