You’re flying along and want to talk to the controller. The controller is already talking to another aircraft, but you don’t hear that aircraft. You key up after diligently waiting a few seconds and then speak. Yet as soon as you unkey the controller is already talking back, but not to you. The controller is talking to someone completely different.
Eventually, the controller is done talking to the other aircraft that you can’t hear, a few seconds pass, and then the controller calls you. But you never heard the other aircraft respond, so how can you tell when to start talking?
