What might cause a VU-9224F 24-button phone to loose it's user's button assignments?
They were all assigned to other user station numbers.

The other VU-9224F phone in the system has had no problems.
The system is about a year and a half old.

Any advice appreciated.

Although I can't say with certainty, it's possible that a dissimilar phone model was plugged into this station port. I didn't think this was an issue with the Summit family of systems, but I know that most prior models of Vodavi products had this issue unless the station lock feature was enabled. I've never had this issue with a Summit, but I suppose anything is possible, especially if DSS consoles are involved.

Thanks Ed,

I'm going to reprogram the buttons and keep an eye on it.
I'll explain to the customer it is very unusual. She may have tried moving her desk phone to a nearby conference table where I have a basic VU-9208 phone.

To refresh my memory, to reprogram the flex buttons for DSS/BLF, is it merely:
1. Press [Trans/PGM]
2. Press desired flex button.
3. Press [Trans/PGM] button
4. Dial [Station #]
5. Press [Hold/Save]

Is there a Program Code needed from Appendix A?

You don't need program codes to just program DSS/BLF buttons. Program codes ARE required if you're programming buttons for particular features, such as paging.