The admin manual is readily available on line but you have to know what telephone system you have not what telephone set is attached to it. You probably have a Comdial DX-80.
The person who recently installed the additional phones unavailable to assist? The voice mail boxes can have the password reset easily but you'll need the voice mail manual for that also. If you don't have the admin passwords you'll have to have a certified tech with the Vertical software on a laptop hookup to the system and read it.
Probably best to have a tech show you the passwords as well as give a quick Voice mail 101 course on mail box admin.
Comdial DX-80/DX-120 Voice Mail Repair/Compact Flash Cards
Comdial Debut Compact Flash Cards