PURPOSE: To shorten a maintenance time by indicating the drop out of an interface unit from a main equipment by respective LEDs when all trunk lines or all push-button telephone sets stored in the interface unit cannot be used.

CONSTITUTION: When the trunk lines L1, L2 are used, if a switch 10 for making busy on the trunk line interface unit 8 is switched, a trunk line interface control part 9 makes the used of the central office lines L3∼Lm which are not used at that time impossible. The trunk lines L1, L2 used at that time cannot be used at the time of the completion of the use and when all the trunk lines L1∼Lm cannot be used, the LED11 for making busy is lighted. When the LED11 for making busy is lighted, the drop out of the trunk line interface unit 8 from the main equipment is indicated for the maintenance purpose.