When the KY-57 is initially turned on (mode switch set to C), an
audio alarm test tone (beep) is generated within the KY-57 and
applied, through part of the received signal path to the handset.
When the handset PTT switch is pressed then released, the audio
alarm will clear. This keying action clears only the KY-57 alarm.
This action does not key the RT. After the alarm has been cleared
and the handset is again keyed, the RT will key.