Abstract: The present invention grants IDs to external lines, and is used to manage whether such lines to which such IDs are linked are under suspension or are being operated. Thereby, even if operational statues have been changed, the identities of the relevant lines can be specified, and the changes of various forms of information related to lines accompanying the changes of line numbers described above can be easily tracked and managed. At the same time, telephone sets to which internal numbers are assigned also store the internal and external correspondence information that indicates what external line numbers can be reached by each telephone set to which an extension number is assigned. Thereby, the operational statuses of internal lines accompanying changes of the operational statuses of such external lines can be easily tracked and managed as needed.