A telephone apparatus that can be connected to both an IP network and a public switched telephone network, the telephone apparatus includes: a microphone unit, a speaker unit, an operation unit that is operated by a user and a call control unit. The call control unit includes a call processing unit and a relay unit. The relay unit includes a call request transmission section; a conversion section; an analog voice data transmission section; and a digital voice data transmission section. Another telephone apparatus that can be connected to the IP network can perform a voice data communication with a public line telephone apparatus connected to the public switched telephone network, via the above-described telephone apparatus.