To prevent a speech of a general telephone set illegally in branch connection to a telephone line to which public telephone sets are connected.
A voltage of lines L1, L2 closed as a loop based on an off-hook operation of a public telephone set is detected and when it is 40 V or below, a transistor(TR) Q1 is turned off to open a DC loop of the lines L1, L2, and in the case that the line voltage after opening the DC loop is 40 V or below, it is judged that a general telephone set 100 is busy and an anti-parallel circuit 12 is used to short-circuit the lines L1, L2. Upon the receipt of a PB signal by a PB receiver during off-hook of the public telephone set, it is judged that the general telephone set makes dialing, the lines L1, L2 are similarly shortcircuited.
TOYODA TAKAAKI
YOSHIDA YOSHIKATSU
MUROI JUNICHI
KAI OSAMU
SATO ISAO
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