When a mobile device falls within a range in which the mobile device can communicate with an IC tag, the mobile device transmits a phone number acquisition request including an IC tag ID and a device number to a management apparatus. The management apparatus receives the phone number acquisition request, acquires a phone number corresponding to the IC tag ID and the device number included in the phone number acquisition request from a phone number management table, and informs the mobile device of the acquired phone number. The mobile device receives the phone number and modifies its own phone number to the received phone number.