PURPOSE: To enable the titled telephone set to be carried to an optional location connected to a general public telephone network or the like and to receive outgoing/incoming call by allowing the portable telephone set to respond to a call from the i-kind exchange and calling the telephone set via the (i+1)th kind broad band exchange station if the telephone set does not reply.

CONSTITUTION: The exchange system is constituted so that the (i+1) kinds (i=1, 2, 3,...) can cover all service areas. The designation information assigned to distinguish the home area resident by plural portable telephone sets at all times and each portable telephone set is registered to the (i)-th kind and the (i+n)-th kind exchanges managing the home area. The portable telephone set is constituted so as to respond to the call from the registered i-th exchange. If not responding thereto, the telephone set is called via the (i+1)th (n=1, 2,...) being a high hierarchy. Thus, when and where the telephone set is available for outgoing/incoming call even in a house, a premises of an exhibition or during walking on a street.