An IP telephone system comprises an IP telephone that performs voice communication via an IP network, a carrier that manages voice communication for the IP telephone via the IP network and a server that returns NAPTR resource record(s) in response to a request from the IP telephone. In the IP telephone system, when the IP telephone performs voice communication via the IP network, the IP telephone transmits, to the server, a request for NAPTR resource record(s) that correspond(s) to the telephone number of the destination terminal. When NAPTR resource record(s) is(are) stored, voice communication is directly performed between the IP telephone and the destination terminal via the IP network according to the NAPTR resource record. On the other hand, when NAPTR resource record(s) is(are) not stored, the carrier manages calls, and voice communication is performed between the IP telephone and the destination terminal via the IP network.