Abstract: A method of improving the CAS (customer premises equipment alert signal) tone signal detection of a telephone which uses a CAS tone signal detection chip or data signal processor in the telephone to detect a small segment of the CAS tone signal received from the exchange line and to trigger a CPU in producing an interruption signal, causing the telephone to mute the voice signal from the near end subscriber and to detect and judge the time series of the left CAS tone signal, and uses a voice signal detection circuit to monitor the voice signal from the far end subscriber, so that the telephone accurately measures the length of the CAS tone signal when no voice signal from the far end subscriber or the near end subscriber is detected, and judges, subject to the definition of a predetermined CAS tone signal length specification, whether the CAS tone signal is true or false.