The system box on the wall should be black. Is there another smaller box of some sort on the wall---or a "PC-like" unit on a desk somewhere? Is there some unit that says "Keyvoice" or "Debut" on it?

How do you access the voicemail? Do you press a button labelled "voicemail", do you dial a station number, or do you get an outside line and dial a number?

There is no "Unisyn" voicemail that I know of, so there must be something else besides your phone system cabinet.