1. A method for configuring a network interface circuit in an integrated services digital network keyphone system, comprising the steps of: first storing in a read only memory (ROM) of the network interface circuit all function programs supporting an integrated services digital network protocol for each country and network which will be used by the keyphone system; selecting a function program corresponding to the country and kind of network to be operated upon initialization from among the function programs stored in the ROM; and setting the selected program, as an execution function program.