Abstract: When adding a new function to a key telephone unit BT, a function addition request is transmitted from a user PC of an user system CM to a customer center CC. The function addition request includes information indicative of the function to be added, a credit number, and a telephone number of the key telephone unit. The customer center CC accesses a credit company CR based on the received credit number, carries out credit research of a user. The customer center CC accesses the key telephone unit BT through a public network PNW based on the telephone number, acquires function information of the key telephone unit BT, judges whether the function can be added based on the function information. If addition is possible, the customer center CC transmits a license code of the function to the key telephone unit BT through the public network PNW, thereby setting the function.