if you search the forum this has been covered in a variety of forms, but always interesting to find out if people have discovered better ways of doing it.
I bought a mono over-ear headset, chopped the plugs off and wired on a 3.5mm stereo jack, works well with my 3100 with "kphone". No good for music, but fine for voice.
I'm going to have a go with a four-pole plug to try and make a stereo headset + microphone Y cable and thus get stereo sound and microphone function at the same time, and put an inline box in the mic lead to make a remote control.