To enable many telephone sets to conduct telephone calls at the same time, with a configuration having few loads related to wiring.
ISDN telephone sets with extended function make a speech with each other, by using a time slot being a division of each B-channel delivered on an extension line to which a voice signal compressed at a high compression rate is set. When an ISDN telephone set makes a speech with another ISDN telephone set via the ISDN, the ISDN telephone set uses a B channel on the extension line to which a voice signal compressed at a low compression rate is set to conduct a telephone call, and a digital service unit 2 relays a voice signal compressed at a low compression rate sent/received through the B channel of the ISDN and a voice signal compressed at a low compression rate sent/received through the B channel on the extension line. Or the ISDN telephone set with extended function conducts a telephone call by using a voice signal compressed at a low compression rate sent/received through a time slot on the extension line, and the digital service unit 2 makes conversion and relay between a voice signal compressed, at a low compression rate sent/received through the B channel of the ISDN and a voice signal compressed at a high compression rate sent/received through the time slot.
MAEKAWA YUJI
KIMURA MASAAKI
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