Thanks so far,
My appology, "doesn't work" means that I expected it to work, because the helicopter socket accepted the plug on my headset, but I could not hear or be heard in the certain helicopters mentioned, where in the others mentioned, it worked perfectly in both regards. All of the helicopters are Canadian operated, and the only non-US manufactured one was the AStar, in which the headset worked! The headset is not noise cancelling, and does have the "US NATO" plug shown, not the stylish "UK Gold" (which must obvioulsy be depicted smaller out of courtesy, or it's attactivenes would completley overwhelm the US one, if compared in the same size image!).
I considered the mic impedance, and it is an older mic, but I do not think that would affect the "phones" function of the headset. The use my headset did not interfere with the aircraft system, as the other pilot could talk to himself over the intercom in each case, and only realised that my headset did not work, when I told him.
One pilot with whom I flew mentioned a ground (earth) difference, but I asked around, and nobody knew. In all cases other than the AStar, the headsets found in, or intended for use in the helicopter were David Clarks, so I just expected mine to work equally well. I will check out the referenced links.
I appreciate your thoughts...
Pilot DAR