Since both of these are ‘Sangoma’ commercial pieces and may be covered under warranty, you might add a second prong to your attach and fire up a support (or maybe issues) ticket.
The error report indicates that it’s just one person and one phone, so I’d start with the ergonomics of the situation. When she reaches for the phone, is there anything that could be bumping the speaker button? I know it’s kind of an obvious question, but you see where I’m going, right? is she using a lifter on the phone, or is she using a headset? Check the case of the phone out and see if the speaker button is ‘captured’ in the down position. Look for the obvious physical stuff. If the button works fine by itself (and she’s not hitting the button accidentally) then try swapping the handset out for another one. If that moves the problem, it’s a bad handset and should (if under warranty) should be replaceable. If the problem persists, the answer has got to be something in her environment.