A voice communication system configured over an IP phone system, with which users could establish voice calls between analog telephone devices and IP phones through an IP phone system without changing their conventional usage method is provided. To allow analog telephone devices to be able to login to the IP phone system, the communication mediation device is required to replace an analog telephone device by be an IP phone user in the IP phone system, and to process voice communications by using analog-digital signal conversion. Using the pre-established registration information, thus allowing the IP phone upon only determining the destination telephone number, it is then capable of automatic converting the telephone number to the corresponding IP phone account so as to proceed with a call through the IP phone system.