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.
AddPac AP-GS1004A is one of the models of inter-network VoIP GSM gateways, through which, it is possible to format a single network unit and to use it for VoIP-telephony, analog communication lines and GSM-network.