Here is a very simple and useful project of a telephone ring detector circuit. The circuit can be used to switch on a loud buzzer when phone rings. It can also be used to switch on some LEDs or a lamp at nighttime if you feel disturbance with your telephone ring noise. One good quality of the circuit is 
is that it will only response on the ring signals and not on other like dial pad tones etc.
The circuit is simple and using only few components. Which are a 2N3904 transistor, relay, photocoupler and few other discrete components. The photocoupler or optocoupler is a device that contains an LED and a phototransistor. When the phone rings then the LED in the optocoupler glows due to which the phototransistor in the optocoupler conduct due to which the 2N3904 transistor conduct and activate the relay. The circuit can be operated with any voltage from 5 to 12 volt DC.

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Telephone Ring Detector Circuit Diagram