Sorry. I saw that your point was missed, but didn't have time to respond in time.

There's not a direct way to do it, but here's a possible work-around:

I'm not sure how this will apply with the C-Class, due to the limitations of the C-Class feature set. Here's a thought I had, though. Setup the MB with no notification, then setup a CFWD-RNA to Guest MB button, where the Guest MB has notification setup (or notif. on MB and none on Guest MB). Also setup a Virtual MB button on the phone for the 2nd MB, so a new message in that box can be seen when sitting at the station.

Now, since I don't remember the feature set limitations, is this even possible on a C-Class?