To recognize all wait times till callback at registration of camp-on.

The number of camp-on registration is accumulated to a multiplex camp-on memory 7 for each of extension terminals 11, 12, 13. Furthermore, a callback sequence message corresponding to an accumulated number of camp-on registration is stored in advance in talkie trunks 2-5. In the case a request of camp-on registration comes from a caller, a central processing unit 6 refers to the multiplex camp-on memory 7 to select any of the talkie trunks 2-5 corresponding to the number of accumulation of camp-on registration corresponding to a called terminal and uses the selected trunk among the talkie trunks 2-5 to send a callback order message to the caller.