What to check
These should be ON dim at idle, and come on brighter when any ringing button is pressed, there are actually 4 LED's in series, if one fails it knocks out the whole row of 4. Apart from not lighting the visitor for the camera, the system should otherwise work.
Corrective Action
Replace the 500-LS pcb.
Links
https://www.entryphone.co.uk/shop/productdetail/PCB-500-LS