Patent ID: 7804828

Claim:
A communication system, comprising: a plurality of telephone terminals; and an exchange apparatus in communication with the plurality of terminals, the exchange apparatus comprises means for transmitting a first packet to at least a first telephone terminal and a second telephone terminal of the plurality of telephone terminals when enabling a telephone conversation to be established between the plurality of terminals, the first packet being used to determine if an intermediary device that alters source address information of transmitted packets is situated between the exchange apparatus and at least one of the first telephone terminal and the second telephone terminal, an information acquisition unit to receive a packet from each of the first and second telephone terminals in response to the first packet and to access (i) source address information added to a header area and (ii) source address information contained within a data area from each received packet, an information recording unit to determine whether the source address information contained within the header area of the received packet from the first telephone terminal has been changed by comparing the source address information contained within the header area of the received packet from the first telephone terminal with the source address information contained within the data area of the received packet from the first telephone terminal, and an information management unit to notify the second telephone terminal if a change is detected and to provide the source address information contained within the header area of the received packet to the second telephone terminal.