Coaches are always hammering goalkeepers to communicate, but sometimes forget that communication is learned just like a technique. We don't expect our keepers to be able to make a diving save without coaching, so why should communication be any different?
Hopefully we are all aware that a call is needed when we are claiming a ball - "Keeper!" Loud, clear, and early. On any cross/corner/free kick when we are not in a position to get the ball, "Away" lets our defenders know the ball is their responsibility.
As we gain experience, it's necessary to start organizing our back line when we can recognize problems. "Step up", "Drop", "Slide left", and "Slide right" get our defenders moving as a unit in front of us. Sometimes we need to communicate with a single player. If that is the case, we need to use their name first, then an instruction. For example, "Bob, step up."
Remember, we are communicating, not commentating. Keep your instructions loud, clear, and concise. The more you use your brain and mouth, the cleaner your uniform will be at the end of the game (hopefully)!