When a CPU receives, from a source IP telephone apparatus, a request “Register (Unregister)” for deleting a
NAPTR record, or when the reception of the request “Register” for registering the
NAPTR record that is regularly received from the source IP telephone apparatus is suspended, the CPU refers to the user data stored in a user
database, and transmits, to an ENUM
server, a request for deleting the
NAPTR record of the source IP telephone apparatus. Further, when a request for the NAPTR
record of the source IP telephone apparatus is regularly received from a destination IP telephone apparatus via a
network interface and when the deletion process of the NAPTR
record is completed, the CPU returns, to the destination IP telephone apparatus, a response indicating that the NAPTR
record of the source IP telephone apparatus is deleted.