Office telephone systems prior to the mid 1970s, were mostly “cloud” based services from the phone company. Private Branch Exchanges, while they did exist, were basically equivalents to the local telephone company.
In the 1960s, Key Service Units or KSUs made their mark in small offices, workgroups, etc. This enabled new technology, such as buzzing, intercom, private lines, and music on hold.
By the 1970s, the innovation of the transistor and integrated circuits (aka computer chips) began to become more affordable for both the vendors and customers. In the mid 1970s, ROLM, Northern, TIE (small end users) and AT&T started to produce electronic telephone systems. Less dependency on hundreds or thousands of copper per site was possible due to using what would become modern phone cords, modular wiring ranging from one to three pairs. The infamous RJ45 or RJ11 plug would become the standard in these setups.
By the 1980s, digital telephony made it’s wave. Office telephone systems at this point became even further electronic and phone systems acted like computer mainframes designed to route data or voice calls. Features like internal caller ID would be seen on digital telephones; the ability to link large mainframes so customers calling a company can process customer information seen on the terminal screen; that could also share the same telephone line used for the telephone was a big deal in the 1980s.
Digital telephones use low voltage wiring and signaling and the large mainframe that would be installed in customer’s plants would be the CPU. All the calls made and received would be processed in these large computers. The telephones on the desk were really “terminals” extending the data carried over the wires that most people would interpret as a “telephone call”. At The Museum, these aren’t “telephones” per se because they can’t be plugged into landlines because they look like phones, but they are much like old VT100 terminals just with a handset and dialpad instead.
Despite the promises of digital telephony, parts and phones were pretty expensive. Line boards could cost up to five grand and “multi line” digital telephones could cost you up to $1,000! For many who spent thousands of dollars of installation of say 1A2 telephones, it was much easier to replace the telco lines and tie it into their digital PBX via an analog telephone card (and some rewiring to make it work for a PBX.) Despite the cost, another reason was the potential difficulty for users to just let go on having to press a line button to begin calling, and the addition of feature keys on these push button sets may had delayed deployment of modern multi appearance telephones.
In the late 1980s, open standards for digital telephony would lead into the Integrated Services Digital Network. ISDN’s max bandwidth was a couple times faster than 56k dial up modems, but in this case this was an “always on” data connection that could carry data, Internet traffic or voice up to 24 “channels” or lines or trunks.
ISDN brought secure, reliable telephony and data services. Or should I say allegedly. ISDN was complex. It’s derogatory nicknames was “It Still Does Nothing” was known for the lack of mass ISDN deployment for the masses. While countries like Europe deployed it even in the age of the Internet (mid 1990s), American customers used it for trunking and for radio programming transmission and for movie trailer voice overs as ISDN provides HD like voice quality without many jitters.
Centrex was still popular to some up till a decade ago. Offices that had Centrex service had POTS phones in the offices, while assistants or receptionist had ISDN telephones similar to the digital PBX telephones previously explained that gave that PBX user experience.
Digital telephony like the 1A2 telephones still exist in buildings that wired for such technology a generation ago. The costs continue to pay off, however maintenance costs may make the reliable technology cost prohibitive requiring them to go to Voice over IP for no real business reasons.