The Number One Electronic Switching System (1ESS) was the first large-scale stored program control (SPC) telephone exchange or electronic switching system in the Bell System. It was manufactured by Western Electric and first placed into service in Succasunna, New Jersey, in May 1965. The switching fabric was composed of a reed relay matrix controlled by wire spring relays which in turn were controlled by a central processing unit (CPU).

The 1AESS central office switch was a plug compatible, higher capacity upgrade from 1ESS with a faster 1A processor that incorporated the existing instruction set for programming compatibility, and used smaller remreed switches, fewer relays, and featured disk storage. It was in service from 1976 to 2017.



Illinois Bell Telephone Company, LLC is the Bell Operating Company serving Illinois. It is owned by AT&T through AT&T Teleholdings, formerly Ameritech.

Their headquarters are at 225 West Randolph St., Chicago, IL. After the 1984 Bell System Divestiture, Illinois Bell became a part of Ameritech, one of the 7 original Regional Bell Operating Companies. The Illinois Bell name continued to be used until January 1993, when Ameritech dropped all of their individual Bell Operating Company names in favor of using their corporate name, Ameritech, for marketing purposes. Illinois Bell started doing business as Ameritech Illinois.

Illinois Bell, in 1971, became the first ever telephone company to offer Call waiting, Three-way calling, Speed Calling, and Call forwarding, all popular local exchange carrier telephone service options to this day.

In 1997, Illinois Bell acquired lines in the Chicago metro area from Central Telephone Company of Illinois, a Sprint company.

In 1998, Ameritech announced intentions to merge with SBC, another former RBOC. Though the merger wasn't well received at first, it was allowed to occur with some conditions. The two companies were officially merged as of October 8, 1999. The Ameritech name continued to be used until 2002, despite the merger.

In 2001, the company started doing business as SBC Ameritech Illinois.

In 2002, SBC dropped the Ameritech name in favor of "SBC" as a national brand. On January 1, 2003, Illinois Bell, for regulatory business, was renamed Illinois Bell Telephone Company d/b/a SBC Illinois. Illinois Bell's trade name was changed once again in 2006, following conclusion of SBC Communications' purchase of AT&T, to Illinois Bell Telephone Company d/b/a AT&T Illinois.