This film explains the Bell System from a blue-collar POV, from a "telephone man" He divides the Bell businesses into 5 segments, and explains what each of them do. It's faux-folksy, but it does the job.
Here are the company segments:
1. AT&T
2. Large local telephone companies (23 of them, like Mountain Bell, Pacific Bell, Ohio Bell, etc)
3. Long Lines
4. Bell Laboratories
5. Western Electric
Most people at the time knew of the Bell System as the company that rented you your phone, serviced the lines and provided basic and long distance telephone service. But the company extended far beyond that into military communications support operations, a number of much smaller subcompanies, and international telephony infrastructure projects.
Footage courtesy of AT&T Archives and History Center, Warren, NJ