PURPOSE:To realize the economical system in which the service performance to the user using a private branch of exchange is in use and the charge of utilizing an ISDN communication network is expected for saving. CONSTITUTION:A service execution section 16 provided to a central processing unit 14 of a private branch of exchange 1 having an I interface in the ISDN connects an information link via an ISDN communication network 15 with an opposite private branch of exchange 2 to implement the service by the common line signal system on the information link. A traffic measurement section 17 measures the traffic of the service by the common line signal system and stores it. A link number control section 18 controls increase/decrease of an information link number with the private branch of exchange 2 based on the traffic measured by the traffic measurement section 17.
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