To enable an extension terminal at one private branch exchange to confirm a conference state of an extension terminal being a speech opposite party at the other private branch exchange in the case of making conference speeches through the use of a digital leased line.
On the occurrence of an execution request of a conference speech using the digital leased line PN1-2 at an extension terminal T11, a private branch exchange EX1 accommodating the extension terminal T11 being a request source notifies a private branch exchange EX2 being a conference speech destination of conference state notification information by utilizing an existing control signal to make transmission reception via a D channel in the digital leased line PN1-2. Then a storage section of the private branch exchange EX2 being the speech opposite party stores the informed conference state notification information, and an LCD of an extension terminal T21 displays DN information of extension terminals T11, T12, T13 in a conference state, denoting that the extension terminal T11 at the private branch exchange EX1 of the speech opposite party is in a conference state.
Sadao Muramatsu
Atsushi Tsuboi
Ryo Hashimoto
Satoshi Kono
Makoto Nakamura
Shoji Kawai