I took the 2.5mm plug off of an etymotic etycom headset and wired it to a 4 conductor 3.5mm plug. I wired the mic (red wire) to the sleeve (pin 4), the ground to the 2nd ring (pin 3) and the earphone (green wire) to the tip (pin 1). Plugged it in and made a call. All seems fine although it's not especially loud in my ear. I wonder if the call app can be adjusted somehow.