"...SAM's "Understanding Telephone Electronics" book ... will teach you the telephone set and telephone line in an excellent manner."

But whose telephone set and whose telephone line?

With telephone stuff, it is essential to understand the specifics of the telephone network of your territory - not just the legal & regulatory stuff but also technical details & practices. All too many books, websites, magazine articles, etc deal exclusively with the North American systems - which are significantly different from many European networks, and other parts of the world.