COMMUNICATION TERMINAL DEVICE AND COMMUNICATION SYSTEM

PROBLEM TO BE SOLVED: To hold high communication quality while preferentially performing communication using an inexpensive IP communication network in a communication system capable of selectively using the IP communication network and a telephone communication network. SOLUTION: When a user senses the deterioration of communication quality in talking between an extension and a connected telephone in a state of connecting the IP network 300 to a line interface 1, the user instructs a change of a decoder by operating an operation part 13. In accordance with the instruction, a line switching processing part 10 transmits decoder information indicating a change from a decoder 6 being used at present to a decoder 7 and a connection request, so that communication using the decoder 7 is established. The decoder information is transmitted to the connected destination and an encoder in a communication terminal device 500 is switched. When the communication quality is still low even after switching the decoder, the user operates the operation part 13 again and instructs switching from the use of the IP network 300 to the use of the telephone network 400 whose quality is guaranteed. COPYRIGHT: (C)2004,JPO