What to check

Find the video splitter port that feeds the affected phone, disconnect the coax and measure for resistance across the coax inner core and the braided screen. The reading should be about 75 ohms which is the terminating resistor in the phone. If the connections in the phone are sound and the resistance check is either open or closed circuit suspect that the cable has failed or is damaged.

Corrective Action

Replace coax.

Additional notes

Beware of tucking excess unsheathed coax behind PCBs as this may cause shorts.

Links

https://www.entryphone.co.uk/shop/productdetail/900-VS4