“Centrex with ESS”, Bell Telephone Company, “New Electric Dial Tone”, Circa 1980s?

“Centrex is a portmanteau of central exchange, a kind of telephone exchange. It provides functions similar to a PBX, but is provisioned with equipment owned by, and located at, the telephone company premises.[1]”

“Centrex service was first installed in the early 1960s in New York’s financial district by New York Telephone.[2][3] As of 2003,[needs update] it was estimated that there were 20 million Centrex lines installed worldwide by 20 telephone companies, with the most installations in the United States (15 million), Canada (2 million), and the United Kingdom (1 million).[2] This accounted for approximately 5% of all installed business telephone lines, worldwide.[2]”