When someone presses the button on Detect Door, the signal is wirelessly transmitted to Call which sounds an alert at the selected frequency and amplification, so you are certain to hear it.The alert sound produced by Call can be set to 16 different frequencies (tones). This means the sound signal can be customised to suit your type of hearing loss. The sound can also be raised to a volume as high as 95 dB SPL. Therefore, you can feel assured that you will know when someone is at the door.