stem 7 network, identifies which carrier owns that specific 800 number. The call is then handed off, and the serving carrier figures out, using its own database, how to deliver the call to its subscriber. Some low-volume 800 numbers are mapped to local tele- phone numbers, but high-volume lines are almost always terminated directly on Divestiture: Equal Access and Chinese Walls the 800 carrier's own switch, typically reaching the customer via special access facilities. The Big Three long distance companies invested heavily in database- driven 800 service, adding features such as automatic call distribution (to divide calls for a single high-volume 800 number among multiple locations) and time- of-day routing. These features were not commercially available from the switch vendors; the carriers did much of their own software development. Large corpo- rate users of 800 service, such as airlines and insurance companies, were their target markets. This feature-driven competition required large sums of capital, but it helped limit the ability of smaller carriers to compete with the majors. Competition led to falling prices for 800 service, leading to more demand, and eventually leading to the exhaust of the 800 code itself and the implementa- tion of new toll-free prefix codes in the 888/877/866 (etc.) series. This was made possible by the 1994 update to the North American Numbering Plan, which made it possible to have area codes without a 1 or 0 as the middle digit. (Technically, the toll-free codes, like premium-rate 900 numbers and other "n00" codes, are not area codes, but service access codes, because they indicate a service and not a geographic region.) The Centrex Revival Centrex, the local exchange carrier service that used a central office to provide PBX-like internal switching functions to a business, had been a Bell System flag- ship offering into the mid-1970s, but its base had been withering away since the Installed Base Migration program to replace most Centrex systems with Dimen- sion PBX. As noted befor