From what I read it looks likes it is when the extension is pressed from the auto attendant.

Before changing any call fowarding on the phone, call the extension from another extension. If it rings, then, as noted above, have the user check in the voicemail box to see if they turned Do Not Disturb on.

On the display, a "CF-NA" is or CF-BNA" is normal, which means call foward is there is no answer of busy, which will likely go to voicemail.