A communication server includes a receiving unit receiving a conversation request signal including a phone number of a receiving terminal in a PSTN (public switched telephone network) via an IP network; a specifying unit specifying transmitting user information on a user using a transmission terminal transmitting the request signal, and assigning an address on the IP network to the transmission terminal; a first transmission unit transmitting the address and the request signal to a communication carrier having a gateway between the PSTN and the IP network; and a second transmission unit transmitting the phone number and the transmitting user information to a transfer server transferring information to a designated terminal.