A communication system for connecting a terminal through a communication channel for a communication signal and a control channel for the control signal is constructed by a decoder for decoding a dial signal such as a push button signal that is generated from the terminal, a transmitting unit for transmitting a dial signal according to an output of the decoder to an external line at the time of a current supply, a power supply unit for supplying a current to a dial signal generator provided for the terminal when the terminal is in an off-hook state, and a setting unit for setting control data for the communication system in accordance with the output of the decoder, wherein when a power failure occurs, the dial signal that is generated from the terminal is transmitted to the external line without passing through the decoder.