The Partner was AT&T's many successors to the Merlin phone system marketing at under 50 phones with Key Telephone System like functionality. This succeeded both Merlin and Spirit, which was another offshoot of AT&T three years before. The Partner released in 1990 with inital builds supporting a handful of telephone lines and about a dozen stations. Over time it succeeded other AT&T offerings and in the Lucent era introudced the Advanced Communications System and Endevavor which was a lower end system (from my research.) More than a decade later, Avaya introduced a 509 control unit, giving customers 5 lines and 9 extensions before an Expansion Module was needed. By 2010, due to greedy Private Equity firms plaguing Avaya, it was discontinued citing supply chains drying up parts. The migration? Repeat after me: IP Office to the Rescue!