A private branch exchange comprising means, including at least an extension terminal, for connecting the extension terminal to a communication network, means for transmitting to the communication network a call including information relating to a caller number, when a transmission call is made from the extension terminal, means for sending, when a call is received from the communication network and the call includes information on a number, the call to the extension terminal corresponding to the called number, memory means for storing information on a predetermined number which is preconfirmed to function as a called number for callback, determination means for determining, when a call has been received from the extension terminal, whether or not the predetermined number is obtained, which corresponds to a caller number received from the extension terminal or a caller number corresponding to the extension number, with use of the information stored in the memory means, and means for employing, when the acquisition of the called number has been determined by the determination means, the predetermined number as a caller number and transmitting transmission call information including information relating to the caller number to the communication network.