One of the earliest digital PBX systems is the AT&T Merlin. The Merlin was initially released just before AT&T's divestiture in 1983/1984 and was meant for very small installations. Earlier digital PBX systems are less well-known because they were much larger.
The capacities of phone systems for very small installations are often denoted in terms of how many trunk/CO or PSTN connections they have, and how many phones (often called stations) can connect to the system. The Merlin initially came in three variants, a 2x6, a 4x10, and an 8x20 configuration. They were the models 206, 410 and 820, respectively.
At the time, the thing that made Merlin special among systems its size was that it was totally digital. The system it replaced was the 1A2 key system. The difference was that a Merlin system used eight wires to each phone and had much less expensive wiring, whereas a 1A2 Key System phone required ten total wires to each phone, making wiring much thicker, more expensive, and complicated.
Today, Merlins are popular both because they're simple and easy to use and install, and because the telephone sets have a certain amount of cultural cachet to them, as they're very visually distinctive, recognizable from media, and if I'm honest, they have a very classy appearance.
The Merlin line expanded upward, with a modular version of the 820 becoming available, and larger cabinet based systems that used line cards becoming available.
The big Merlins were replaced by the Merlin Legend and Merlin Magix systems, which were expandable, modular systems that allowed for a customer to choose what connectivity options they wanted to have available and expand the number of stations. At the very low end, the Partner system initially allowed for five lines and nine phones, and could be placed in a carrier for expansion with more modules.
The AT&T Spirit is another, non-expandable that was available in the late 1980s and early 1990s. Its big feature would have been that it was inexpensive, which was almost certainly the tradeoff for expandability. It's not clear today (at least to me, as of this writing) how all of these products relate not only in the product stack, but chronologically. If, for example, Spirit, Partner, and Legend/Magix were all shipping at the same time, how were they related as products? The Legend and Magix were pretty clearly larger, modular and expandable systems with a variety of options such as DS0, PRI, and BRI connectivity. How were Spirit and Partner related, though? Was Spirit a successor to the Merlin 206 and 410, with barebones options and no expandability with Partner being an expandable, more featured midrange option? Or, was Spirit the direct successor to the lowest end Merlins, and Partner the successor to that?
It would be nice to have a little bit more context, both when looking for stations to attempt to use with different systems (if I should happen to buy a Partner, for example) and just because it's nice to know. We know the details of Apple's and IBM's product lines, why not AT&T/Lucent/Avaya?
In the early 2000s Partner, Legend, and Magix were all ultimately succeeded by the IP Office platform, which supported some of the same digital phones as the previous systems, but with added IP connectivity to new IP desk phones, as well as adding other connectivity options, such as SIP trunking, allowing a business to connect small remote offices to a larger system over the Internet, or to use the Internet to establish local phone numbers in multiple area codes, without adding new wiring.
Later on in the 2000s, after the acquisition of Northern Telecom, the IP Office 500 rolled both Nortel and Avaya systems into a single unit, and switched "expansion" from line cards or blade-style modules to modules (which were essentially rack-mounted servers that you could set on a table mount to a wall, or put in a rack) that connected together with Ethernet.
Today, the IP Office platform still exists, but Avaya is very coy about exactly how small it can get. There's an IP Office Basic Edition, but compared to the IP Office Small Office Edition from 2003, it's absolutely massive, supporting 20 SIP trunks, 32 analogue lines, a T1/E1, and up to 100 stations. There's no word from Avaya on whether or not they consider this system appropriate to install in locations that need fewer than ten total phones, which is an environment that may still need conferencing, calling from line to line, voicemail, and so on, but where installing the current IP Office hardware, which involves a regular rack-mount server from HP or Dell, may not make sense.
It's possible that Avaya is ceding this market. Somewhat ironically, one of the more active players in the market for very small phone systems is AT&T, with their Syn248, SynJ and Synapse systems. These are built (with AT&T's logo on them) by V-Tech and seem to be one of the more heavily marketed options for a small plug-and-play phone system these days.