I don't know the answer, which doesn't help you. However, phantom power for microphones can be as high as 48V, so be cautious. Perhaps you could take a voltmeter with you and simply measure the voltage across the microphone pins of the headset connector.
But I would heed Fursty Ferret's warning - the last thing you need is smoke in the cockpit or a loss of communications through using non-approved home made equipment
