PURPOSE: To make it possible to communicate by connecting an extension without inversion of a trunk line when a received special number sides perform back communications by storing the connection information for special numbers such as 110, 119, etc., from an extension terminal and storing the information of the extension.

CONSTITUTION: When special numbers such as 110, 119, etc., are dialed from an extension terminal, a central control part 11 searches a vacant circuit of trunk circuits 31a-31n and transmits the dial information on the special numbers to a trunk line using this circuit. On the side of an office, the special number of this dial information is called A control part 11 registers in a storage part 12 that a special number connection was requested and makes the part 12 store the special number connection information. The control part 11 monitors a trunk circuit for the time when transmissions are performed from the sides of special numbers and performs the monitoring of incoming without inversion from the trunk line by referring to the storage information.