The first on site customer systems was the PBX, but later on in the 1920s into the 1930s, brought a specific solutions to customers, called the Key Telephone System.
The “key” to this was to have hard wired lines for users who wanted to get to someone directly. This also fused with Intercom systems, acting as a separate “service” for the time. Intercoms for this context was a private circuit to enable telephone calls to work within the system. Most often, the Intercom line was on the fifth button of say a 564 telephone.