Here's a new one to us:

Customer reported a certain 9508 is missing the "Hold" key when a 2nd call starts ringing through. Apparently if the user presses "Answer" the first call is immediately disconnected. All the typical settings existing under Telephony - Reserve Last CA, Call Waiting, Answer Call Waiting on Hold, etc.

We tried replacing the set and changing the port but neither made a difference. It was finally resolved by deleting the user and re-creating it again.

The focus is moving to the ringing call, the one starting with 587 (notice the call appearance is highlighted)

Scroll UP to the current call, the one starting with 403, and you will find the Hold button that is active for that call.

IF you have Auto Hold checked in System > Telephony, pressing Answer on the ringing call that is in focus will put the active call on hold automatically.

On the phone, press Features > Call Settings... > Auto Display Waiting Call > Set to OFF and the focus will not move from the active call to the ringing call.

This value is saved in the IPO's configuration, not in the telephone itself, which is why replacing the phone did not change the behaviour, but deleting and recreating the User did.