KEY TELEPHONE SYSTEM AND MEDIUM RECORDING MASTER SET CONTROL PROGRAM

PROBLEM TO BE SOLVED: To allow a user using a terminal adaptor to efficiently utilize a B channel of an ISDN line through the connection of a plurality of personal computers. SOLUTION: A personal computer 103 is connected to a serial interface 18 of a terminal adaptor 102, which has a PPP/MP processing section 17 that conducts asynchronous/synchronous PPP conversion and MP processing. A master set 100 is provided with a router section 20 consisting of an external line PPP/MP processing section 7 that conducts PPP, MP processing for external line communication, of an extension PPP/MP processing 8 that conducts PPP, MP processing with the terminal adaptor 102, of a router function section 9 that conducts router processing, and a router main control section 10 that controls each of the sections 7-9. When receiving a call request from an extension telephone set 101 during communication through a 2B channel of an ISDN line by the MP communication, the router section 20 releases a 1B channel in 2 sets of B channels during the MP communication. A main control section 5 assigns the relieved 1B channel to the extension telephone set that makes a call request.