At least the way it's set up in our office, dragging something from one Category group to another adds the new category, but doesn't delete the old -- so it stays in "@Calls" even though it's now also added to "@Office." To get rid of the old category you have to open it up or, as the responders here pointed out, right click.

I'm not complaining about this feature of categories. It's great to have a field that supports multiple entries for some purposes -- it's a powerful feature of Outlook's database -- but it doesn't work for GTD contexts.