The Bell System History book 1925-1975 explains all this in detail.
> [TELECOM Digest Editor's Note: Lisa, where would one obtain this
> 'Bell System History book' to which you refer? PAT]
I think she's referring to "A History of Science and Technology in the Bell System," which is a multi-volume set published around 1985. It's in the University of Georgia library and probably many other libraries.