With an idle few minutes I took a screwdriver to a 9508. The initial 4 screws (one at each corner) are obvious and easy enough. But then it becomes clear that Avaya have joined the two halves at another point. From my inspection I suspect a heat weld between the two halves, located in a dimple roughly in the centre of the phones. base. You probable could remove that with a bit of careful drill work but I wasn't going to go that far with my non-sticky phone.
So meanwhile, clean the phone with a wet wipe, dry it and give it back to the user telling them to be more careful (but most importantly mark it clearly on the base so when it turns up on someone else's desk you can swap them back).
Stuck in a never ending cycle of file copying.