This invention relates to a communication system using an internet protocol (IP) address. Subscriber terminals have the same subscriber identification and different IP addresses. A server establishes a connection between the terminals via the network. A service system has a table including the subscriber identification information and the IP addresses of the subscriber terminals.When a connection request is made by a subscriber terminal to the group of subscriber terminals while a connection between one subscriber terminals of the group and another subscriber terminal is established, the service system detects an unused IP address associated with the common subscriber identification information of the group of subscriber terminals in the table and notifies the unused IP address to the sever, and the sever makes a call to a subscriber terminal in accordance with the unused IP address.