They type "look behind you" because they are dead and you can't hear their voice chat.
I have found that 1 in 5 players prefer to communicate with voice but being the only guy doing it sucks so they don't speak up.
If you set the example and consistently lead the way in using voice chat properly, they will join in. Those are the type of players who we want coming back to the server. They are looking for a server where they can have an engaging and fun fight.
This is why one of the server rules is "No Mic Spamming". I usually silence players who are being obnoxious in voice comms because it counters everything we are trying to do. Players shouting useless nonsense about "snackbars" etc are discouraging legitimate players from speaking up or sticking around at all.
Interestingly, once you silence the spamming player and they realize they can no longer fill our ears with their diarrhea-of-the-mouth, they voluntarily leave. (usually after intentionally TK'ing on their way out the door and proving they're a shitty player)