Is it easy to change the main company voicemail greeting on my NEC phone system?

Yes, you can easily record a new main company voicemail greeting or toggle to a prerecorded message from inside or outside the office. That makes it easy to change your main greeting when you close your office for bad weather.  For Example, the NEC SL9100 allows specific users to be designated as administrators and they can change the main system greeting from their mailboxes.