| 摘要 |
In a telephone system 1 connectable to both integrated service digital network (ISDN) lines 3a and 3b and a public subscriber telephone network (PSTN) line 7 and having a plurality of extension telephone sets 11a to 11d, the speech sound volume through the ISDN lines 3a and 3b is so adjusted as to become equal to that through the PSTN line 7, by changing the loss rate (i.e. gain) of digital pads 55 and 47 provided in ISDN interface units 5a and 5b. That is, a level confirmation tone is generated in the system 1; this tone is transmitted to a first extension telephone set 11a via only the ISDN on one hand and to a second extension telephone set 11b via both the ISDN and PSTN in series on the other hand; and the loss rate (i.e. gain) of the digital pads 55 and 47 are changed or adjusted so that the sound of the level confirmation tone can be heard at the same volume level by both the telephone sets 11a and 11b.
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