System 75 and System 85 were AT&T’s large digital PBXs of the 1980s: cabinets full of circuit packs, administered from a text terminal. DEFINITY carried the architecture through the 1990s, and it survives today as Avaya Aura Communication Manager.
What callers heard
Music on hold came from an external source (a tuner, a CD player, or an announcement board) wired to an analog port and administered as the system’s music source. There was no factory song. A station’s Class of Restriction even decides whether its held callers hear music at all.
Communication Manager kept the same idea. We found no bundled default track for it either.