| 摘要 |
PROBLEM TO BE SOLVED: To surely detect ring-off of an opposite party. SOLUTION: When an externally mounted telephone set off-hook detection section 31 detects on-hook of a call return reply telephone set after an emergency notice in the ring-off detection method, a control section 11 activates a relay RL1 and acquire a line. A dial tone DT detection section 15 detects a dial tone after the acquisition of the line. When the DT detection section 15 detects dial tone, the control section 11 can discriminate that there is ring-off.
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