Please check the following.
● If the indoor unit has a power button, check that the power is on.
● Check that the ring volume is not the lowest.
* You can adjust the ring tone (chime sound) and screen brightness with the "adjustment knobs" installed on the side or bottom of the device.
If there is no improvement after adjusting the volume, check the manufacturer / model number of the monitor phone and let us know through the "Inquiry Form" or contact the service center.