Well, per the user's guide, to turn off the feature and the lamp you 1) press the feature light off key then 2) lift the handset or press the "Handsfree Mute" key or press the "dial" key. It says nothing about entering a code.
Does this not work in your situation?
Regards,
Harry
"The legitimate powers of government extend to such acts only as are injurious to others. But it does me no injury for my neighbour to say there are twenty gods, or no god. It neither picks my pocket nor breaks my leg." --Thomas Jefferson