Circuit shows a phone busy indicator circuit. The telephone lines also possess polarity. SO, to make the circuit to work without polarity issue, a bridge rectifier is added to the circuit. Diodes VD1-VD4 forms the polarity conversion circuit, which makes us not to be worried about the polarity of the telephone line. The circuit working is stated as follows: When the telephone is at hook, the circuit will be a D.C circuit, and so, the the thyristor VS remains at Cut-off state. When any incoming call arrives, the AC ringing voltage changes to 60V, which supplies a breakdown voltage to the regulator diode(VD5).The diode regulator voltage is 12V. When it happens, LED goes to ON state. This indicates that the phone is busy.