Hold key operation enhancement

This feature enhances the existing OpenStage phones by providing a single Hold/Retrieve FPK which can be used to hold and retrieve calls. In addition the LED associated with the Hold FPK will be used to indicate that a call is currently being held.

Current releases of OpenStage SIP Phones support a Hold FPK but the functionality is limited to simply placing the currently active call on hold. The implementation of this feature enhancement extends the functionality of the Hold FPK on non-keyset devices in the following ways.

On a keyset enabled phone the FPK can be used to place a call on manual hold but the associated LED will not indicate the hold status and the FPK cannot be used to retrieve the held call. This is because on a keyset device the FPK associated with the line is used to indicate the hold status and to retrieve the call. This is consistent with the implementation on the optiPoint. It is currently not possible to allocate the Hold function to a shifted FPK and this will still be the case following the implementation of these enhancements.
 * 1) The FPK can be used to both hold and retrieve a call.
 * 2) The LED associated with the FPK will be used to indicate that a call is held and can be retrieved by pressing the FPK.  The LED will be illuminated whenever the retrieve option is available to the user.