Didn't help, but thanks for trying.
The 'faulty' handset has a similar display to the other two in that the standby message displays the ID of the handset. Specifically, the problem handset displays HANDSET 1.
If a number is dialled on a working handset, I get a dial tone and the dialled number stays displayed (rather than the handset ID) until the call is answered at which point the display becomes a call timer.
On the faulty handset, as soon as the dialled number is 'sent', no dial tone is heard and the dialled number is replaced by the handset ID, HANDSET 1.