| 摘要 |
In a private branch exchange system which includes a private branch exchange accommodating a plurality of line wires and extensions for switching lines between the two, and telephones, which are usable in the event of a power failure, connected to the extensions, the line wires and the power-failure telephones are connected when power feed to the private branch exchange is no longer performed owing to a power failure or the like. When power feed is restored, the call status of the power-failure telephone is monitored and, if it is found that a call is in progress.
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