I have an AT&T Merlin 820D2. There is 1 Line Board and 2 Station Boards installed. I haven't worked with these very much, would this be a considered a 4x20 system?

SOURCE: How do I set up voicemail in merlin 410 system?

MFurrer is right... the voicemail unit was made by Voysys. It is known by several names: Merlin Classic Mail, vp100, model 100.
Some telcomm suppliers who deal in used or refurb equipment MAY still have one or you can contact King Technologies (they own the rights to this unit). Either of these is likely to give you "sticker shock" ($1000+). Ebay is about the only cheap place to find one, but there are always buzzards circling over these (myself included). If you find one, make sure it includes the power supply. The units on ebay have several typical problems: bad hard drive, no power supply or deleted admin mailbox. All these should be deal-breakers unless you're planning on buying several for parts. The deleted Admin box is the only one that can't be discovered until the unit is attached to phone sys. It is fixable but the people who know how don't seem to want to share this knowledge.
Jim

SOURCE: Number of Rings Before Call Goes To Voicemail

From the system programming menu either in the pc programming tool or from the console phone enter into system programming, options,more, group coverage delay. set here from 1 ring to 9. Good luck.

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Thanks Dan