PURPOSE: To attain the communication between extension terminal equipments accommodated to different exchanges connected in multi-stages by sending a line fault alarm signal synchronizingly to the exchanges connected in multi- stages from the exchange having a fault when the fault takes place on a digital lien interconnecitng to a communication network.

CONSTITUTION: When a fault takes place on a digital line 7 connecting to a digital communication network 9, an alarm signal representing the line fault is sent to exchanges 2 of multi-stage connection connecting to an exchange 1 without out of synchronization via an alarm signal transmission means 1b. Thus, even when the fault takes place in the digital line 7 connecting to the digital communication network 9, the communication between extension terminal equipments 3, 5 or between extension terminal equipments 4, 6 accommodated in the exchanges 1, 2 of multi-stage connection is implemented.