NEC UX5000 Feature Handbook page 132

Reverse Voice Over
When You're On a Call
To place a Reverse Voice Over Call:
1. Press and hold FUNCTION
Your Reverse Voice Over key lights (green).
To return to your initial call:
FUNCTION
1. Release
When Your Terminal is Idle
To place a call to your Reverse Voice Over destination:
1. FUNCTION (Reverse Voice Over).
You can optionally lift the handset for privacy.
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(Reverse Voice Over).
(Reverse Voice Over).