| 摘要 |
PURPOSE:To attain bypass connection even if a public network line connected directly to a private branch of exchange is faulty by connecting private branch of exchanges by a private line and relaying a call from a private branch of exchange via the private line. CONSTITUTION:In applying the exchange connection connecting a telephone terminal set 3 to a telephone terminal set 4 in a public network under the control of a private branch of exchange 1, the private branch of exchange 1 applies relaying to a call from the telephone terminal set 3 via a private line 2 and the private branch of exchange 1'' to apply succeeding connection to the exchange 5'' connecting directly to the private branch of exchange 1'' to make a call to a telephone terminal set 4. As a result, even if a transmission line between the private branch of exchange 1 and the exchange of the public network is faulty, the telephone terminal sets 3, 4 are connected and the private branch of exchange network is utilized as much as possible effectively without using the public telephone network to the utmost.
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