If you can't talk, son, just key your handset twice. Over.
5 seconds sound clip from the Platoon movie soundboard.
You can hear this line at 01:37:07 in the Blu-ray version of the movie.
Quote context
[...]
- Just tell me where the rounds hit. Over.
- How about those rounds, son? Can you adjust fire? Over.
- If you can't talk, son, just key your handset twice. Over.
- Trip flare! Rodriguez's hole!
[...]