The Merlin systems were basically from the beginning designed to be a workgroup phone system to compare it to the computer networking world. While the Merlin phones look so big business, and in some cases they were. Small Key units like the Merlin were installed in large environments against existing Centrex and electromechanical or analog/digital PBX systems. Because those systems already had 8 or 9 for the outside line, this would be redundant and therefore the Merlin did not have this feature. For small setups it was easy to pick up the phone and make a call. However this one and succeeding phone systems, many central offices would get quick off/on hook statuses because the users would be making an internal call. One trick was to hit the Intercom on hook then pick up the set.
“Feature modules” were these sliding cartridges that would enable users to do more features. While I personally never used a Merlin, it’s believed these were add ons, and some were actual software improvements, while some were value added features.
AT&T marked the living crap out of this system as this knocked the socks of many small businesses and consumers. In fact Merlins were sold in their Phone Center Stores, a chain of brick and mortar stores that also sold Mickey Mouse and Snoopy telephones at your local mall, and their Sourcebook catalogs. And some famous CEOs loved the innovative system so much, they had them in their house well into the 2000s. Famous writers as well. The telephones were made at the Shreveport Works plant at AT&T, while other components were made elsewhere. In fact so many assume the Merlin brand was also synonymous to the other PBX and telsets.
Despite the “classic” 1983 Merlin being electronic, it was not digital, nor did it support things like time of day, calling line ID, and so on. In 1987, AT&T made such things possible as they introduced their first digital phone system for small businesses called the Merlin II. The system was similar to the 1070, but had some improvements to the design. The circuit boards for the II were enclosed in plastic, so the chances of ESD would not cause harm. While it supported the classic Merlin sets, the goal was to push for new digital sets, like the lone 7406 that was borrowed from the System 75 telephone product line. No other DCP set was supported. At the same time, a design similar to the II but based on the analog/classic system, the Merlin Plus was released in the same year.
Another offspring was the Merlin Plus released in late 1986, it’s control unit was modular, but the circuit boards were bare. The “classic” design was not used.
The closest to PBX functionality in the Merlin class was the System 25, using the similar System 75 circuit boards. The processing on the inside was similar to the Merlin system. The terminal interface for changing and adding extensions wasn’t as friendly as the System 75 and required one of their own UNIX workstations to do the changes. The System 25 seemed to not have that much of a longevity compared to the other PBX systems and even the Merlin systems themselves in terms of sales and marketing and support. It’s official end of sale is not to be found.
A common misperception is the “Merlin” sets had a similar look to their larger cousin, the 7400 DCP telsets that were used in larger System 75 and 85 sets; which often caused people confusion of the Merlin being the “big system” or the DCP sets were known as “Merlin sets.” One reason for the confusion was the Merlin sets were also supported for that “investment protection” some may had been recycled to an upgrade.
But in fact, you could tell by looking at the side of the unit (say the original Merlin and 7400s) and if you saw a single “AT&T” emblem, it could be anything other than a Merlin, because the outline typeface “MERLIN” would indicate such in the same area. The Multi Line Extended (MLX) sets marketed in the early 1990s, also mimicked the design of the 8400 sets of the said larger systems. It’s very easy to just assume these were “Merlin phones” too.
The Merlin competed not just itself, but two other lines, the AT&T Spirit (1987) and the AT&T Partner (1990), succession to the Merlin II, the Legend (1990/91). Often AT&T introduced new features to the original 1983 Merlin system, such as faxing adjuncts requiring a 34 button telephone acting as the keypad and phone functionality. In the early 1990s they band-aided the original system and phones with a caller ID unit that clashed with the original Merlin look with an AT&T home answering machine like form factor. Then another backwards support: voice messaging – voiced over no other than the lady behind AUDIX. AT&T wasn’t like Nortel (or even Apple) where they felt they needed to outdo the previous generation. In fact Avaya’s own brand was on some of the very older AT&T DCP and Merlin telephones up till late 2001.
The peak of the Merlins could very well be the mid 1990s, because in the early 2000s, the Merlin (albeit the “classic” editions) started to fade away. Given parts failing, and the very expensive nature of third hand resellers, the Merlin would concede by the end of the last decade to hobbyists, etc.