PURPOSE: To identify the terminals that belong to an interface group of a representative group when a packet call is received by the representative group of a private branch switchboard from an ISDN.

CONSTITUTION: When a packet call is received, the D-channel call control signal procedure SQ is carried out between a private branch switchboard 2A and an ISDN 1. Then the switchboard 2A checks the normal state of an interface group a1 of a representative group 3A. When the normal state of the group a1 is confirmed, the TEI check request messages are sent to all terminals of the group a1. Meanwhile each terminal sets a packet/non-packet terminal identification code in an information field of a TEI check answer message and then transmits the identification code to the switchboard 2A. If a packet terminal is connected in the group a1, the switchboard 2A connects the call and transmits a calling packet.