If your system is an ADIX, the display setting is global in Class 4.01 as you noted. You can test the display of the phone itself by pressing FEAT #, 9 at any phone, then press a button. The display will show how it's programmed, verifying the display is functioning. KTD-3 phone displays are field replaceable, and sometimes work themselves loose. Try unscrewing them, removing them and re-installing them. Unplug the phone first....

If your system is an ECS, all of the above troubleshooting applies, however there is programming that allows the phone to either display system names or station names. That's how the series 1 phone was displaying what it was....that's found under Stations>Display or Class 70.07.

Sometimes the thoughts in my head get so bored, they go for a stroll through my mouth. This is rarely a good thing.