There is a feature in many panel programs that monitors the voltage of the phone line it is connected to. If the voltage of the phone line drops and the system is armed at the time, the siren will ring locally as a notification to you that your phone line may be down. If the siren is not armed, just the “trouble” indicator will beep. You can recall the “trouble” condition and confirm that there was Telephone line trouble. If the panel is downloadable, we can switch this feature off if you prefer. Check your user manual regarding the “Trouble” light.