Patent ID: 7599358

Claim:
A telephone system, comprising a plurality of telephone sets having unique address information on a packet communication network, an exchange device which controls incoming calls to the plurality of telephone sets on the basis of the address information and a server device connected to the packet communication network, wherein each of the plurality of telephone sets comprises a notification unit that accepts a user log-in operation based on identification information of users and notifies the identification information of users and address information of its own telephone set to the server device in a registration request, and the server device includes: a user management table associating each telephone number of the plurality of telephone sets with the identification information of the users; an address management table dynamically associating each telephone number of the users with address information of the corresponding telephone sets; a specification processing unit which specifies a telephone number of a logged-in user by referring to the user management table on the basis of the identification information when the log-in operation is performed; a verification processing unit which verifies whether the specified telephone number is registered by referring to the address management table on the basis of the specified telephone number; a determination processing unit which determines whether the address information of the registered specified telephone number and the notified address information are the same by referring to the address management table, if the specified telephone number has already been registered; and an update processing unit which updates the address management table by overwriting address information of the registered telephone number in the address management table with the notified address information, if it is determined that the notified address information is not the same as the address information of the registered telephone number, wherein the server device notifies the registered telephone that it has been deregistered when is determined that the notified address information is not the same as the address information of the registered telephone number and that the registration request is an initial registration request.