Does this happen only when the user presses the Messages button on the IP phone? Can the user from another phone, such as cell phone, dial into Unity access number and sign in to access voicemail OK? If so, I would suspect the IP phone's Calling search space or partition is not correct.
If you said the user wasn't getting voice messages, I would have you check the voicemail profile on the IP phone, directory number configuration. But you said that is working. I think having the user confirm access to voicemail from another phone, i.e. cell to test Unity's pilot number and sign-in process will eliminate Unity from the problem you are having. Also, trying dialing the Unity pilot number from the problem phone to see if Unity answers. You should get the Opening greeting.
These are the paths to get to each CCX logs through CLI. They may be helpful if you are having issues accessing RTMT or downloading logs through it.
If you want to download them you have to prefix "file get " and you can add one of the options (re...