Standard Horizon VLH-3000 Owner's Manual page 9

Figure 3. Controls and Switches
MICROPHONE
Speak into the Mic while the PTT button is press to transmit audio to the
PA horn or the Intercom Speaker.
CALL Button
Used only when a Intercom Speaker is connected to IC1 or IC2. When
pressed produces a beep in the connected Intercom Speaker.
VLH-3000 Page 9