| 摘要 |
PURPOSE:To improve the utilizing efficiency of a relay line by sending a busy tone to an incoming line connected to an outgoing line from the inside of an exchange station to restore immediately the outgoing trunk connected to the outgoing relay line when an outgoing exchange detects a busy tone returned from the outgoing relay line. CONSTITUTION:When a called telephone set TELB4 is busy, an incoming exchange EXB3 connects an incoming trunk ICT32 and a busy tone sender BT33 to return a busy tone. The busy tone reaches a calling telephone set TELA2 via a relay line, an outgoing trunk OGT12 and a channel switch SWA11 and inputted to a busy tone detector BTD14 at the same time. When the BTD14 detects a busy tone, the detected output allows a CPU15 to switch the channel connecting a TELA2 and the OGT12 to the connection of the TELA2 and the BT13. Then the TELA2 receives a busy tone of the outgoing exchange EXA1, the OGT12 reaches the same state as the cutoff of the TELA2, a cutoff signal is sent to the EXB33 for cutoff. When a cutoff end signal is returned from the EXB3 to the OGT12, the OGT is restored and the relay line is used for the next talking.
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