e in Chapter 2, the postdivestiture RBOCs, with no installed base of PBXs, took a very different view of Centrex and once again made it their flagship offering. Bell Atlantic was especially dependent on Cen- trex revenues; its flagship account was the federal government, with millions of lines in the Washington area and beyond. NYNEX in New York and Ameritech in Chicago were also major Centrex markets. BellSouth may have been some- what hurt by its earlier regulatory success, as its regulators allowed many of its old Centrex tariffs to be replaced with a costlier substitute called ESSX; most regulators in other regions had been less inclined to accept that predivestiture proposal. But by 1984, Centrex had a serious technical disadvantage compared with most modern PBX systems. Almost all computer-controlled PBXs, including the analog Dimensions as well as popular digital systems such as those from ROLM, Northern Telecom, and NEC, had electronic telephone instruments that provided one-button access to common features, such as call transfer, hold, and conference calling. Even more important, they provided support for The Great Telecom Meltdown multiple line appearances on a single electronic telephone set, providing for features such as secretarial coverage of a group of users. This was possible on predivestiture Centrex but only by using multiline key telephone sets (line- selection buttons being known as "keys"). These typically used 25-pair or 50- pair wiring to the desk, leading to extensive relay-based control systems called key service units (KSUs) in wiring closets. Electromechanical key sets were very labor-intensive; adding or moving a line required rewiring at the KSU. Elec- tronic sets, in contrast, needed only one to three pairs of standard telephone wire and could be reprogrammed from a system console or computer terminal, often remotely. They also provided extra features, like caller identification (if coming from another extension of the same PBX system), that Centrex could not yet provide. Northern T