The headset on the N900 uses a 3-ring, 4-contact plug. I am trying to figure out what contacts carry what signal for a project I am doing. I am pretty sure the Left Audio Signal uses one contact, the Right Audio Signal uses another, the mic should use the third and I would think the fourth would be a ground. But if my theory is right what controls the "end call" button on the headset (next to the mic)? Any ideas?

The project is basically that I am putting a female 4-contact receptacle in the center console of my car and wiring it to my factory stereo, adding a mic to the headliner so that I can play my mp3's off the N900 over my car stereo and take/make calls just as I would if I was wearing the headset. If anyone knows how the end call button is wired up please let me know.

I found this pinout which doesn't address the end call button issue at all:
http://pinouts.ru/HeadsetsHeadphones...e_pinout.shtml

Thanks!

Try using a multimeter to measure the resistance of the mic contact vs GND before and while pushing the button. I bet it's going up or infinite (circuit break).

Breaking left or right audio would be bad for the user, because it disrupts audio. Breaking gnd would do the same. So you're up with the mic contact there. As your plug scheme also indicates it is used for c-video via multiplexing, means there is some more higher logic elements in the n900 attached to that one.

PS: Know your multimeter, don't kill the mic with it

I just tried recording audio from the mic, it went mute while the button was held down; Renkon's guess seems to be correct, the button cuts the mic.