Throat Mic for Moto t5420

Anyone know if you have to have a special motorola throat mic. I purchased a couple off ebay with moto single pin that are VOX. Plugged them in to our T5420 and the VOX on one radio holds open the tx and the other can rx but can not tx since the other radio in open tx even when no one is speaking.

I would have thought if the throat mic is VOX and it works with this moto (this model has VOX capable) it should work. Am I missing something. Help