Old books but good ones
Interested in 1970s telephony? Who isn't? Hayden book author David Talley penned three good titles back then: Basic Carrier Telephony, Basic Telephone Switching Systems, and Basic Electronic Switching for Telephone Systems. I have the first two. You can probably find them all at http://www.abe.com (external link.
How many telephones are in the world? I have no idea. AT&T's Long Line Division was the only group who had a good estimate. Each year they published a thin book on world telephone statistics. It was called, with variants, The World's Telephones. I think the last was published in the mid-1980s. You can also find it at abe.com. Some interesting figures from the 1975 consensus: Japan increased its phone tally from 12,250,841 in 1965 to 41,904,960 in 1975. That was a 242% increase. The U.K. more than doubled their phone population in the same era. 9,960,00 in 1965 to 20,342,457 in 1975. Quicker computerized switching and automating operator duties made higher call volumes possible. I say that because I doubt Japan or England doubled or tripled the amount of their physical plant during that time, rather, it was the efficiencies of the computer age that caused the increase.