I made an adapter that I use with a Heil headset and also a computer headset. If you look at the mic pin connections in the 817 manual pg 5. Pin 4 is the mic audio input and you need to install a 4.7k ohm resistor to pin 6 ( marked +5v) to provide power for the mic element. The same adapter works for both the Heil and the computer headset mic.
If I remember correctly the numbers on the plug pin out in the manual is reversed fron a cat5 pin out.
