PURPOSE: To transmit quickly information by identifying additional dialing of a prescribed number with a prescribed register to send a call waiting tone to an extension subscriber if an originating subscriber dials additionally this prescribed number.

CONSTITUTION: When an extension subscriber 6a is busy, a central controller 5 connects a terminating central office line trunk 2 to a talkie trunk 4a through a switch network 1. Then, the trunk 4a sends information, which indicates that the extension subscriber 6a is busy, and a message of an additional dial number for call waiting to a trunk line. When the office-side originating subscriber dials the call waiting number, the dial number is counted by a PB receiver 3 to send information to the controller 5. Thereafter, the call waiting tone is transmitted from a tone trunk 8 to the subscriber 6a.