Abstract: A telecommunication jack box with a trouble-shooting capability is disclosed. The telecommunication jack box is characterized in that it has a test receptacle including a switch mechanism disposed therein and that it is separably installed at a location adjacent to a place where the telephone line enters the house or the building and in the meantime at a location in the vicinity of the telephone set. The test receptacle serving the purpose of determining whether a phone set and the incoming line it was used on are operable. As a result, the telehpone system remains in an operational condition even at the time when the indoor telephone line is under repair. Furthermore, the telecommunication jack box can be used as an ordinary telephone jack box.