Your siren may not be programmed in correctly. Ensure that the siren ‘beeps’ when transmitting a ‘tamper-off’ signal from the panel. If the problem persists, try re-‘learning’ in your siren unit, ensuring that the master/slave switch ‘4’ is set to ‘off’ and that you hear a confirmation ‘beep’ during transmission of the ‘learn siren’ signal from the control panel.