 Why did AT&T originally retain ownership of the phones at the endpoints?
Trains and planes, telegraph and telephone, all brought many people into the same causal space. The solution to this increased complexity in the late 19th, early 20th century was to increase the role of structure and improve its design. During the first two-thirds of the twentieth century, this type of rationalization took the form of ever-more complex managed systems, with crisp specification of roles, lines of authority, communication and control.
 To ensure the proper functioning of the networking and monitoring of customer behavior.
 To prohibit customers from connecting unlicensed phones at the endpoints.  To exclude competitors.  To ensure the proper functioning of the networking and monitoring of customer behavior.  To make the most profit.