The Bell System
The Bell System Memorial is the most comprehensive site around on the old AT&T telephone monopoly. Its capsule history of AT&T is outstanding.
AT&T Long Lines
The long lines history at Bell System Memorial is excellent. Another excellent resource shares more details on the microwave towers that were the heart of the long-lines network after the 1950s. For the broader context, ColdWarComms.org details how the long lines system fit into national security plans.