When pressing 'CAPS LOCK' to speak via radio the audio should have all high and low frequencies removed leaving only the harsh mid 500-1000Hz (rough guess)
So that while inside loud vehicles or during heavy battles the voice will still cut through over the engine noise or gun fire nice and clearly.
There should also be the classic:
(Press to talk) initially emits a 'blip' sound
(Let go when finished talking) emits a 'CH!' sound
The players natural voice should still be emitted from the characters mouth, so when talking to team mates on a radio channel, players without a radio (or enemies on a different channel) can hear the character speaking as if on direct communication.
VBS Radio: The next wave in radio communications for VBS3
Here's another great example in the game 'Squad'...