try speaker 45, or 43 then 200. you need the voicemail pilot number which is usually 200 but could be anything. try the code at a phone the works and the display should show you the pilot number, then do speaker 45 or 43 plus pilot number and hangup
You need to do it from an admin mailbox and should call an NEC vendor to help if your not sure what it is. When in admin mailbox press MGR then INSTRU then 001 mailbox is day and 002 mailbox is night