Ha - Ok ! - I remember now - ! - sorry about that
Here is the procedure for the Non-Display OpenStage 10
Plug in the phone to a working Digital Line
'The operating mode of an OpenStage 10 phone is changed via the 3 function keys.
1. Initially, all 3 LEDs of the function keys are off. Press the rightmost function key for at least 5 seconds.
2. The current active phone operating mode will be indicated by the LED on one of the other two function keys.
When OpenStage mode is active, the middle LED glows by itself, whereas in OptiPoint mode the leftmost LED glows by itself.
To toggle between modes press either the leftmost or middle function key depending on which mode is currently active.
From OpenStage to OptiPoint mode - press the left most key
From OptiPoint to OpenStage mode - press the centre key
For completeness - the Switching Procedure for OpenStage 15/30 is:
1. Press the menu button (three parallel lines) for at least 5 seconds.
2. The menu LED starts flashing and the phone mode selection menu is shown on the display. The currently active phone mode is shown.
3. Press the Up or Down/left or right arrows to change mode - when you have the correct mode selected press the 'OK' button to store.
4. Press OK again to confirm the selection and activate the chosen mode - after 5 seconds the phone will perform a restart and will start up in the new mode.