Analog wired Headset
There's a four-ring 2.5mm stereo jack which provides connectivity to old-fashioned wired headsets.
The headsets used by Motorola smartphones (A780,A1200, ...) and the V-360 have a compatible configuration.
The headsets used by some Nokia phones (N95, ...) have the same jack but they are incompatible. For technical information about rewiring them visit Analog wired Headset Nokia
2: left out
3: right out
4(tip): mic + HOLD-button(press:short to GND)
base = ground speaker left (internal impedance 33R) to ground. (+jackinsert detection) speaker right (internal impedance 33R) to ground. tip = mic electret condenser type, to ground. bias (power for mic) 2K2 from +3.3v(wolfson codec) (+HoldButton shortcircuit to ground)
Dylan proposed a proof of concept scripts to fix something about the headset insertion/removal being incorrectly acted upon.