A telephone sends, in accordance with a user's instruction, a call connection request that includes a source telephone number and a destination telephone number to a connecting device or VoIP server device that corresponds to the source telephone number. Management server device, upon identifying a set of a source telephone number and a destination telephone number that is more appropriate than a set of those included in the call connection request, sends a notice to the telephone to prompt a user to change the telephone numbers. The connecting device or VoIP server device that establishes a call connection in accordance with the call connection request, notifies a destination telephone of a to-be-notified telephone number that is selected by the management server device based on the destination telephone number.