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.
A modern voice communication SIP protocol is widely used to make calls in the Internet. However, the final subscriber is mostly interested not in the very SIP standard but in the SIP telephony conception based on it.
One one hand, all traditional conveniences for making telephone calls, compatibility with the international subscription base and the procedure of dialing numbers are kept.