An authentication system is applied for a voice network in order to provide the same security as that attained by a data network. Provided is a voice network access system wherein a one-time password system, which is used as an access authentication system, is applied for a voice network that is constituted by an private automatic branch exchange that includes an external interface; wherein the private automatic branch exchange is connected to a computer telephony server across the external interface for the exchange of sender number information that is received from a public network; and wherein the private automatic branch exchange is also connected via the external interface to an authentication system server, which receives a user's name from the computer telephony server and performs a personal authentication process using the user's name.