Here's my guess.

Charlie was broadcast through a video feed in a communication with Penny just before he died.
Boone was talking through a 2-way radio to the survivors of the tail section just before he died.
I'm not sure off-hand with Libby, but I'm guessing there may have been a similar circumstance.

In the event any of those broadcasts were ever intercepted, it covers up their tracks by having already indicated those people had survived the crash but died afterwards.