The headset receptacle is a 3.5mm audio socket with 4 poles. The poles are arranged in a way allowing for standard unmodified stereo headphones to be used.

The pins are:

• 1 (tip) left HP output
• 2 right HP output
• 3 GND
• 4 (sleeve) microphone

AC100 can use a regular electret microphone connected to the 4th pin.

NB: some noname 4-pole jack i bought for a DIY adapter was impossible to insert before filing its tip.

This arrangement is compatible with the numerous iPhone headsets and is not compatible (without hardware mods) with the scheme used by Nokia.