| 摘要 |
PROBLEM TO BE SOLVED: To maintain the speech of a slave device for power failure at the time of operating power resupply to a main device, and to validly use a plurality of speech channels even in a power failure. SOLUTION: A line switching device 1 respectively switches and connects a plurality of speech channels of an ISDN line 10 to either a main device 2 side or slave devices 30A and 30B for power failure side. The slave devices 30A and 30B for power failure start a speech by using any of those speech channels of the ISDN line 10 through the main device 2 in a conductive state, and start a speech by using any of those speech channels of the ISDN line through the line switch device 1 in a power failure state. When the power failure state is switched to a power supply state, the main device 2 detects the using situation of each speech channel of the ISDN line by the slave devices 30A and 30B for power failure through the line switch device 1, and confirms the available speech channel.
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