What to check

Inside the telephone check the curly cord connector lead. It is a four-wire (red, black, yellow and green) assembly connecting the cord socket at the bottom left of the phone and the phone PCB. Try moving the wires to check for an intermittent or poor connection particularly at the PCB end.

Corrective Action

Replace 'patch lead'

Links

https://www.entryphone.co.uk/shop/productdetail/200-500-PLD