In November 1960 C. Breen and D. A. Dahlbaum of Bell Labs in New York published "Signaling Systems for the Control of Telephone Switching," Bell System Technical Journal, 39 (1960) 1381-1444.
"Telephone signaling is basically a matter of transferring information between machines, and between humans and machines. The techniques developed to accomplish this have evolved over the years in step with advances in the total telephone art. The history of this evolution is traced, starting from the early simple manual switchboard days to the present Direct Distance Dialing era. The effect of the increasing sophistication in automatic switching and transmission systems and their influence on signaling principles are discussed. Emphasis is given to the signaling systems used between central offices of the nationwide telephone network and the influence on such systems of the characteristics of switching systems and their information requirements, the transmission media and the compatibility problem. A review is made of the forms and characteristics of some of the interoffice signaling systems presently in use. In addition, the problem of signaling between Bell System and overseas telephone systems is reviewed with reference to delivering information requirements, signaling techniques and new transmission media. Finally, some speculation is made on the future trends of telephone signaling systems" (abstract of the paper).
According to http://www.historyofphonephreaking.org/docs.php, the Breen and Dahlbaum paper is
"often cited as the article that gave away the keys to the kingdom," leading to the development of the underground "phreaker" culture. Other papers that included the in-band trunk signaling tones which provided the technical information needed to build Blue Boxes are cited at http://www.lospadres.info/thorg/bstj.html, accessed 09-17-2009).
My thanks to Jeffrey Odel for this reference.