To relieve the cost burden on a user, to decrease a revision required time, and to reduce the cost and labor in a customer center.

In the case of adding a new function to key telephone systems BT1 to BT4, first a user PC of a user system CM transmits a function addition request to a customer center CC. The function addition request includes information denoting a function desirably to be added and a telephone number of the key telephone system. Then the customer center CC accesses the key telephone systems BT1 to BT4 of the user on the basis of the received telephone number via a public network PNW to acquire function information of the key telephone systems BT1 to BT4 and discriminates the propriety of the added function on the basis of the function information. When the addition is available, the customer center transmits a license code of the added function to the key telephone systems BT1 to BT4 of the user via the public network PNW to set the addition of the function.