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.
“Call filter” allows blocking/unblocking the incoming calls as easy as to make filtration, it also journalizes all rejected calls. It is very convenient if you do not want to receive a call from some subscribers, when they will call you, “Call filter” will do so that your line is engaged.