| 摘要 |
PURPOSE:To visibly and audibly display the absence of a called key telephone set operator to an originating key telephone set operator automatically by providing each telephone set operator automatically by providing each telephone set with a circuit which decides the installation environment of a called telephone set from the quantity of its ambient light. CONSTITUTION:A photosensor 14 is fitted on the console panel, etc., of a button telephone set and senses the light in the environment of the installed key telephone set to send its level signal to an environment decision part 15. The environment decision part 15 stores data 'bright' when said level signal is higher than a preset level or 'dark' when not in a memory 13. When one key telephone set calls another telephone set by an extension call, the main control part 11 of the called TEL reads the data out of the memory 13 and sends out data indicating absence to a main device through an interface 12 when the read data is 'dark'. Consequently, the main device performs control for sending out an ordinary tone indicating the absence to the originating telephone set.
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