I noticed this accidentally when I tried to lock my PTT to always transmit, during an encounter, so I would not have to hold down an extra button while talking.

However, double pressing the VoIP-Button only caused the game to switch from PTT to Voice Activation, which was horrible, because it was cutting out all the time. Please return to the previous way. If you need to add a switch to toggle in-between both modes, rather add an additional hotkey. However, I don't see any benefit for a way to quickly switch from PTT to Voice Activation or vice-versa in the middle of the game.