What's the easiest way to add voice mail to a Merlin 820 system? Is there an internal module for that, or is there some other method?

Thx

AT&T used to sell A Voice mail called a classic mail. I was also called a Voysys.

First off if you get one itwill be close to 10 years old.
Second it uses 4 of yout extention ports (one of which is 10)
Third It is acient technology. They Barely integrate with the merlin and I would not sell one to any on my customers (even ones I don't like I want to get rid of)

You would be much better off selling them A Partner, Vodavi STS, or an NEC DS2000 or any other system you sell

Actually, I'm not a reseller, I'm supporting our Church - we were donated an 820 system several years ago and it work fine - except that we have an analog answering machine attached to our 1st line (before it enters the Merlin) - so its only good when nobody is on the 1st line. Its a fully loaded system with all 10 lines and 20 phones capable, but we only use 3 lines and 5 or 6 phones.

What options (preferably cheap) do we have to add a basic voice mail to this system?

WHY DONT YOU BUY 2 MORE $25.00 ANSWERING MACHINES FROM COSTCO,BESTBUY ETC. AND PUT THEM ON OTHER 2 LINES OR TAKE ADVICE ABOUT PARTNER ETC.

just subscribe to the local phone company voice mail. It will be your most cost effective method for you.