A telephone applicable to PSTN and IP network having functions of obtaining, when a call is originated by entering a PSTN telephone number, an IP network telephone number of a destination telephone corresponding to the PSTN telephone number from a telephone number translation server connected to the IP network, obtaining IP address information of the destination telephone corresponding to the IP network telephone number from a call agent connected to the IP network, establishing call connection with the destination telephone via the IP network by using the IP address information, and establishing call connection with the destination telephone via the PSTN if the IP network telephone number of the destination telephone cannot be obtained from the telephone number translation server.