All extensions are unable to access VM via the MSG button. Only the group dicando VoiceMail (eg 4444).
Was this working before or is it now broken?
As for the phone with the light stuck on, I'm now thinking you may have to delete it and re-create it.
But, you could also try renumbering it then then activating the lamp like you did earlier and turning it off again. If it goes off then you can just renumber it back to what it was. If it doesn't go off then delete it and recreate it again.
Ralph