A test device of an AD converter includes a compare decision circuit for comparing the output code of an AD converter with a specified digital code representing a bit transition point, and for generating a digital signal corresponding to the difference between the two codes compared. The first device is configured to produce primary power through the primary coil upon being supplied with external power and to be controlled by a provided control signal so that the primary power decreases and increases upon the control signal representing a first value and a second value, respectively. The system affects call transfer based on priority state.