Nokia brand handsfree unit of the HS47 type is an integrated unit with two earphones, a call-receive button and a microphone hidden in the central plastic unit, and a 4-contact 2.5mm jack. This kind of a jack is becoming a sort of standard in the world of cellphone handsfrees.
The unit itself is fairly simple. The earphones are connected directly to the tip and first ring of the jack, against the ground. The microphone is connected via a simple RLC filtering network, to filter out the RF noise of the cellphone transmitter. (I suppose this filter, together with the overall quality of the transducers, is the main difference against no-name handsfrees.) The parts used are L01 and L02 (20 µH each), a capacitor whose value could not be reliably measured but is guessed to be about 2 nF, and a part labeled V01 which is unknown but may be a 1.42 kiloohm resistor.
The call-receive button connects a 47 ohm resistor across the microphone (R01). (The microphone is electret, powered from the phone via an internal resistor, in a way standard for such microphones.) This causes a drop of the microphone voltage, which is detected by the phone as a signal. The resistor probably serves as an overcurrent protection.
Opening, cover removed
Opening, button removed
System board, top