| 摘要 |
PROBLEM TO BE SOLVED: To prevent a base unit and a line from being connected while a branch telephone is being used. SOLUTION: When the off-hook of a handset 150 is detected (step S1), a control part 100 decides whether the branch telephone is in use on the basis of the voltage detection of a branch line L2 by a voltage detecting part 115 (step S2). When the branch telephone is in use, the connection of a line L and a base unit 10 is refused by maintaining the OFF state of a switch 120 (step S4). At the same time, a message which shows that the branch telephone is in use and which is stored in a storage part 180 is sent out of the handset 150 (step S5).
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