Abstract: A telephone circuit is composed of a current circuit, an hybrid circuit, a noise inhibiting unit, and a CAS detector circuit. The current circuit has two input ends connected with tip and ring of the telephone line, and two output ends connected with two input ends of the hybrid circuit. The signal received by two input ends of the hybrid circuit is analyzed. The noise inhibiting unit is connected in series between the hybrid circuit and the CAS detector circuit and is provided with at least one inhibiting circuit which has two input ends connected with the first signal line and the second signal line of the hybrid circuit. The output ends of the inhibiting circuit are connected with two input ends of the CAS detector circuit for minimizing the error of identifying the CAS signal by the CAS detector circuit.