A copy & paste of my my Skype forum reply here:
“Guys be careful, there is only ONE headset that is supported in the dualphone.
I spoke to the Product Manager of RTX because they did not know themselves that most other headsets WON’T WORK because there is no standard for 2.5 mm chinch phone headsets and many will not work for this reason-.
I tried it and it involved a lot of phone calls and crap, etc. Had to return it, etc, all the hassle and anger just to find out that only one is supported.
Unfortunately the headset is no longer produced and cannot be bought in many countries (e.g. Germany).
Tried it, gave up.
Here is it:
Jabra EarWaveBoom – 2.5mm universal (part number: 100-73030000-50) Go to http://www.jabra.com or follow this deep link: here.
All those info are from RTX Product Manager with asking the technicans that developed the phone“