An ENUM server stores NAPTR records including PSTN telephone numbers and the corresponding IP telephone numbers of a source IP telephone apparatus and a destination IP telephone apparatus. When the ENUM server receives an ENUM query from the source IP telephone apparatus, the ENUM query setting a domain name based on the PSTN telephone number, the ENUM server searches for a NAPTR record corresponding to the domain name and returns, to the source IP telephone apparatus, an ENUM response including the NAPTR record. When receiving the ENUM response from the ENUM server, the source IP telephone apparatus retrieves the IP telephone number of the destination IP telephone apparatus, the IP telephone number being specified in the NAPTR record contained in the ENUM response, and starts a call connection sequence with the destination IP telephone apparatus.