PURPOSE: To prevent a TEI from being double registered at the plural ISDN terminal equipments.

CONSTITUTION: An ISDN terminal equipment 13 is equipped with a ROM 14, and this ROM stores random number initial values peculiar for the respective terminal equipments. When the terminal equipment is started, a CPU 13b applies the random number initial value to a random number generation part 13d as a random number generating command. While responding to the random number generating command, the random number generation part 13d applies a random number to an ISDN exchange 11 as a TEI allocating request. Since each terminal equipment generates the random number based on the random number initial value, the TEI can be prevented from being double registered.