During the Kundang Camp level, you will eventually end up chasing Sadono underground. Your CO will advise you that they will no longer be able to communicate with you while you are underground, due to interference from copper.
However, if you get caught by a guard or an alarm is sounded, your CO will be able come over your earpiece saying that you need to watch the alarms. The female counterpart will also say something, even though they are not supposed to be able to reach you.