NEC Electra Elite IPK II System User Manual page 142

Document Revision 4C
If you do the following in sequence,

Set Call Forwarding Busy on extension.

Set Call Forwarding Immediate on extension.

Cancel Call Forwarding Immediate on extension.

then,
Call Forwarding Busy/No Answer is set back on extension
Default Setting
Enabled
System Availability
Terminals
Any Station and Virtual Extensions
Required Component(s)
None
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Electra Elite IPK II
Call Forwarding