PURPOSE: To realize a connection function improving the degree of freedom of the channel selection of a trunk line connected with a public network and adopting a unique function for ISDN packet calling, that is, multiple connection function on the identical channel and a D channel.

CONSTITUTION: A trunk line channel selection information circuit 1 reads out the channel using state of the corresponding trunk line from a trunk line channel resource management circuit 3 when a trunk line channel selection request is received. A channel to be used in the trunk line is decided based on the priority order shown in trunk line channel selection data 2 stored in the circuit, and the result is transmitted as a trunk line channel selection response.