used this pricing flexibil- ity to encourage their retirement. This created a boomlet for the PBX industry, as the bulk of the remaining electromechanical and precomputer PBXs were replaced. Between 1975 and 1985, practically the entire installed base of PBXs in the United States had been refreshed. From that time forward, PBX sales were keyed to more natural factors, such as economic growth. And small flurries of replacement activity occurred again in 1994, when the North American Numbering Plan introduced inter- changeable area codes (those without a 1 or 0 as the second digit-some older PBXs, including some remaining analog Dimensions, could not accommodate the change), and again in 1999, as the "Y2K" craze struck. The PBX industry as a whole survived these disappointments, for the simple reason that there remained some need for their services. But its boom years of 1975-1985 were never repeated, and such a boom is unlikely to be repeated. MCI's Shared Microwave Opened New Doors If private companies could own their own microwave networks, then why could not more than one company share a network? That was the basic idea behind Microwave Communications, Inc. (MCI), founded by William Goeken in 1963 with the stated aim of building a shared microwave link between Chicago and St. Louis. Of course a shared network is what a common carrier provides, SO AT&T naturally opposed this. The idea languished at the FCC until 1969, when MCI was granted permission to provide leased-line services. And with that move, the cracks in the monopoly armor began to grow wider. Shortly after- wards, MCI was granted permission to attach its lines to Bell System local tele- phone lines. MCI became a formidable competitor, in the courtroom as much as in the field, and the "natural monopoly" argument began to fall apart. Private Line Competition Led to Rate Restructuring By authorizing MCI to provide leased lines (more often, in those days, called private lines), AT&T faced its first serious common carrier competitor in dec- ades. AT&T's p