| 摘要 |
PURPOSE: To avoid mistake in on-hook/off-hook detection of a branch telephone set by discriminating a voltage change by a voltage detection circuit even under an environment such that a line voltage is changed slowly or unstability is caused due to the effect of noise. CONSTITUTION: In the communication terminal equipment in which a line voltage detection circuit 6 is connected in parallel with a telephone line 2 which a branch telephone set 3 is connected, a sampled line voltage V(n) and a line voltage V(n-T) sampled at a preceding fixed time T to the sampled time are sequentially compared for every sampling of the line voltage and a state change of the branch telephone set is discriminated when the result of discrimination that the voltage difference is larger than reference levelsα,βreaches consecutively a prescribed number of times.
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