Nokia HS-40 is a cheap mono headset that has a 2.5mm jack with 4 rings. 
It costs 9 EUR in spain. The pinout is INCOMPATIBLE with OM, hence nokia 
pinouts are incompatible with OM. Here it is the Nokia HS-40 pinout:

Nokia HS-40 pinout:
1 (tip)  - left (R=34 ohm to ground)
2        - unused (in stereo models, probably connects right)
3        - mic (mic resistance 1K1 to ground, if button is pressed 48R)
4 (base) - ground

>From this message, here it is the OM pinout: 

Openmoko pinout:
1 (tip)  - mic (2K2 ohm and 0R if button is pressed)
2        - right
3        - left
4 (base) - ground

If we plug the headset in the OM, only one speaker would work (i never 
did it). This was reported by David Pottage in this message: 

I stopped measuring and began hacking ;-) The little box which has the 
mic and the button, has two screws. If you remove them and open the 
case, you'll see 3 wires + 1 wire shielding coming from the jack. It 
should be 4 wires + shield, but nokia removed the unused wire from right 
speaker. Get a multimeter and find the wires that connect with pin 1 and 
3 of the jack. Take a solder iron and swap wires from 1 and 3, and you 
have a headset with a pinout compatible with OM.

I plugged the headset in the GTA02 and loaded the gsmheadset.state 
during a call. I only had to increase the gain of the output from 80% to 
95% because the output from speaker was very low. 34R is below the 40R 
minimum impedance that joerg recommended in his message. Microphone is 
ok and people heard me fine. GSM buzz from 
https://docs.openmoko.org/trac/ticket/883 is still randomly heard by the 
other end (slight in local).

Button doesn't activate anything in the GTA02. I don't know if it 
should. The 48R smd resistance can be easily transformed into a 
shorcircuit if needed.

I have a few photos, i could create a nokia headsets section in the wiki 
if you think is useful.

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