how can i restore the caller display on my bench kh5001 comfort telephone?

For no apparent reason I have lost the "caller number display" facility on my BENCH KH5001 analogue comfort telephone, I have replaced the batteries, enabled the facility using *#234# after first contacting BT. There does not appear to be a fault on the line. I have replaced the phone and the replacement phone works with the facility. I have reset the KH5001 back to the factory default settings by removing the batteries for more than 10 mins. Reconnected the phone but fault is still present.