XPR4550 Mic Sense Pin 1

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hello, We are trying to figure out how the XPR4550 knows the difference between the normal microphone, the smart Mic and the Handset. We want to fool the radio into thinking it has a handset to do some on hook off hook audio reverting.
I tried measuring pin 1 to ground for the smart mic vs the regular mic and neither has a resistor to ground.
FYI, during tests we discovered the "admit criteria" on the tx side of the channel info causes the rx side of the muting to act in different ways. To have open monitor when off hook the admit has to be "always" !!