Method of controlling silent monitoring of a group

A system for silent monitoring of a group, said system having a silent monitoring function which allows a monitoring individual to monitor a call between an individual to be monitored and a customer, said system comprising:

• a first ISDN exchange accommodating terminal of monitored individuals;
• a second ISDN exchange accommodating a terminal of monitoring individual;
• and
• an ISDN trunk line connecting said first and second ISDN exchanges;
• wherein said first ISDN exchange forms the monitored individuals into groups, allocates monitoring target groups to the monitoring individual and observes call-in-progress/call-not-in-progress status of the monitored individuals group by group;
• when a monitoring request has been issued by the monitoring individual, said second ISDN exchange sends said first ISDN exchange a call set-up request message, which includes the identification number of the monitoring individual and a monitoring start request;
• and
• said first ISDN exchange receives the call set-up request message and sends a call signal, which is obtained from a customer and a call-in-progress monitored individual belonging to the monitoring target group allocated to the monitoring individual, to the terminal of the monitoring individual via said ISDN trunk line and said second ISDN exchange.