An intercom system to be used in a housing complex constituted by a plurality of dwelling buildings (A-E), wherein each of the dwelling buildings (A-E) comprises: a dwelling parent device installed in each dwelling, a management office parent device installed in a management office of the considered dwelling building (A-E), the management office parent device being communicatively connected to the dwelling parent device via an intercom line (LI, L2) and capable of responding to a calling from the dwelling parent device, and a building control unit communicatively connected to the dwelling parent device of each dwelling in the considered dwelling building (A-E) and the management office parent device in the considered dwelling building (A-E) via the intercom line (LI, L2), wherein the building control units of the plurality of dwelling buildings (A-E) are interconnected via a network on which communication is to be performed, wherein the network is further connected with a centralized building (F, G), and the centralized building (F, G) comprises a centralized management office parent device capable of making a phone call to the dwelling parent device in each dwelling of the plurality of dwelling buildings, and a centralized control unit communicatively connected to the centralized management office parent device via the intercom line, the intercom system being characterized in that the building control unit in each dwelling building (A-E) is configured to enable the setting of a specific calling time zone in which it is possible to call the centralized management office parent device or the management office parent device of another dwelling building (A-E), in addition to the management office parent device in the considered dwelling building (A-E), and when there is a calling from the dwelling parent device of a dwelling building (A-E) in the set calling time zone, the management office parent device in the same dwelling building (A-E) is configured to receive a calling signal from the dwelling parent device and the building control unit is configured to receive the calling signal from the dwelling parent device and to transmit the calling signal to the centralized management office parent device or the management office parent device of the other dwelling building (A-E). a dwelling parent device installed in each dwelling, a management office parent device installed in a management office of the considered dwelling building (A-E), the management office parent device being communicatively connected to the dwelling parent device via an intercom line (LI, L2) and capable of responding to a calling from the dwelling parent device, and a building control unit communicatively connected to the dwelling parent device of each dwelling in the considered dwelling building (A-E) and the management office parent device in the considered dwelling building (A-E) via the intercom line (LI, L2), wherein the building control units of the plurality of dwelling buildings (A-E) are interconnected via a network on which communication is to be performed, wherein the network is further connected with a centralized building (F, G), and the centralized building (F, G) comprises a centralized management office parent device capable of making a phone call to the dwelling parent device in each dwelling of the plurality of dwelling buildings, and a centralized control unit communicatively connected to the centralized management office parent device via the intercom line, the intercom system being characterized in that the building control unit in each dwelling building (A-E) is configured to enable the setting of a specific calling time zone in which it is possible to call the centralized management office parent device or the management office parent device of another dwelling building (A-E), in addition to the management office parent device in the considered dwelling building (A-E), and when there is a calling from the dwelling parent device of a dwelling building (A-E) in the set calling time zone, the management office parent device in the same dwelling building (A-E) is configured to receive a calling signal from the dwelling parent device and the building control unit is configured to receive the calling signal from the dwelling parent device and to transmit the calling signal to the centralized management office parent device or the management office parent device of the other dwelling building (A-E).