The AT&T Merlin 410 phone system was manufactured in the 1980's and some are still going strong (not like the new equipment made these days that will not last 5 years let alone 30). On top of the Merlin systems going strong, the phones are tanks and are still used in thousands of businesses across the United States.
That being said, when your Merlin 410 KSU gets hit by lighting, the warning light comes on, and/or there was a power surge and your system is no longer functioning what is the next step? AT&T created the Merlin Plus phone system after its Classic series (206, 410, 820). These Merlin Plus phone systems were designed that the small business had more flexibility. They were also made better. Overall, the Merlin Plus system experiences far fewer issues than its predecessor. You can replace your Merlin 410 phone system (206 or 820 for that matter) with the Merlin Plus phone system and keep the same phones.
The Merlin Plus system has a built in feature module so you will not need the feature packs that are traditional to the Merlin classic phone systems. So, if you are unable to find an exact replacement, feel confident in getting the Merlin Plus system as a replacement.