In the early days, there was just one phone company – Ma Bell (AT&T Long Lines). But starting in the late 1960s there was competition. And it cost AT&T the monopoly on both local and long distance services. The phone system has never been the same since.
AT&T Long Distance Network
The original Ma Bell (the nickname for AT&T) Long Distance Network
See also the History of AT&T Long Lines
See also the modern AT&T 4ESS/N4E Tandem List
See also AT&T Historical Videos
MCI and Sprint
Both MCI and Sprint provided competition to Ma Bell. This competition led to the eventual breakup of the Bell System as we knew it on January 1, 1984. However, neither of these companies exist anymore. Here’s their history and what led to their respective downfalls.
The History MCI Long Distance Company (Now a division of Verizon Business)
See also the Unofficial MCI Tandem List
The History of Sprint (The former Long Distance, Local and Wireless company)
See also the Unofficial Sprint Tandem List