There are 3 radio channels built into the game.
Squad leaders can communicate to one another and the team commander by holding down G. This is reserved for squad leaders.
Squad members can talk to one another holding down B.
Then you can talk to anyone on your team within 25 meters by holding V. This is not a radio channel, but rather you just basically talking in the open. As such, there is no accompany radio squelch, and sound occlusion effects kick-in. (So, the further away a friendly is, the fainter your voice sounds. Or if you’re in another room, your voice is going to sound muffled to your teammates, etc, etc).
The preferred method of communicating is to avoid using B unless you need to talk to the entire squad, or a squad member you need to talk to is too far for V to be used. The reason it’s generally good protocol to keep the radio clear, especially for the squad leader. If everyone is yelling on the radio, and the squad leader is also trying to talk to other squad leaders, it’s a lot to process.
Note, that you can use local chat (V) and the enemy cannot hear you if they are nearby, so you don’t need to whisper. But they can hear your footsteps, or broken twigs if you’re moving in the bush, or your gunfire, of course.