I tried hard to manage how to change system keys labels (like call key) and didn't find. I don't think it's possible. I was able to rename feature keys (like conference, system speed calls calls, etc.)...
You can't have a "release" key on the 5000 as the 3300/2000/200. Each time we replace a Nortel system for a 5000, users are complaining about that but they seems to get used to after few days... Some users use the "button" under the handset as a release key and are happy with that.
Hope that helps a little bit!