This circuit goes between a telephone and a headset.  The project has a headset cord and plug (cut from a cheap cell phone headset) on the input and a suitable jack on the output.  The speaker signal passes through this circuit, but the microphone signal is passed straight from the plug to the jack.

This circuit is specifically designed for ultra-low battery consumption.  It uses a special control circuit (Q3, Q4 and !5) to regulate the power amplifier's quiescent current.  Also, effort was spent to design an auto-on circuit (Q10 through Q14) that draws only 1.5 uA, yet has 15mV sensitivity.