A 2N5777 photo-Darlington cell picks up blinking light from the transparent plastic buttons. The power is switched ON and OFF by a hi-beta 2N3904 transistor. The circuit's 9 V battery can be left continuously connected. Less than a micro-ampere is drawn-even with normal, office ambient light and the phone lights not flashing.
For noisy locations, the tone can be made louder with an output transformer (ratio of 250:8) or a 100 ohm speaker that replaces the 22 ohm resistor in the output.