Most likely the voicemail system you have is called Elite unless you were sold a 3rd party system.
If it is the Elite NEC installer would have had software on his or her laptop called cosessions, this software is used to configure the voicemail system, you will need to call around your area for NEC dealer or reseller and hope they have a old laptop with this software along with the know-how.

Do you know what mailbox was given admin rights? If so, it can't hurt trying random numbers for passwords. (No limit on number of tries).

We get old too soon, smart too late