Universal Apple-compatible CTIA Headset/Earphone 3.5mm Jack (LRGM) pins and signals

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Warning! Control buttons are NOT standard. iPhone/Pad/Pod control buttons do not work anywhere else. Android control buttons don't work anywhere else. For information about control buttons take a look to another pinouts

Headset Button Wiring

Button is activated by shorting the mic input pin(4) to ground(3) (for HTC hero headset and some others). The function depends on the resistance between this short. Direct short is the play/pause/take call button,short with a 600ohm resistor and it is the play next function, short with a 220 ohm resistor and you have the play previous function. For other models still unknown.

Cellphone external input pinout

To detect and use an external input, the Samsung Galaxy S needs to see about 1600 ohms impedance between the microphone and ground pins. This will automatically disable the internal mic in the phone. To connect the output from a PC or other audio device into the phones mic input, you'll need to add a 1.6k ohm resistor in series. This is the same procedure that works with an iPhone. 

Standard stereo earphones with 3 connections instead of 4 will work to listen to music.  That's because standard 3 pin 3.5 mm stereo earbuds have the same pinout as above, except that there is no pin 4.  That means that pin 4 is the same as pin 3, which means the microphone input will be grounded, which will work fine for listening to music.

Headset compatibility issues

However, a standard headset with one earpiece and a microphone will usually not work when plugged into these cell phones.  For example, the Plantronics M214C headset will not work with the Samsung Moment or HTC Hero unless you make a special adaptor cord (using a simple standard 2.5 to 3.5 mm adaptor cord will usually not work).  That's because the Plantronics M214C has this pinout on it's 2.5 mm plug:

Pin 1 TIP: Mic
Pin 2 Ring: Speaker
Pin 3 Sleeve: Ground