Abstract: Accordingly, the present invention relates to a monitoring system and method for a trunk gateway. In the monitoring system of the present invention, a trunk gateway is interposed between the Internet and a PSTN and is adapted to packetize a trunk line into packets and to transmit the packets to the Internet, or to perform a reverse procedure. The monitoring system includes a VoIP agent connected to the Internet and adapted to originate and terminate a VoIP call and to generate an MOS value using a voice file received from a counterpart. A circuit network agent is connected to the PSTN and adapted to originate and terminate a call and to generate a MOS value using a voice file received from a counterpart. A center server performs various types of control operations related to measurement of service quality of the trunk gateway, including establishment and termination of a call, and collects the MOS values.