"same principles for every telephone of the world"

Granted - the principles are the same; but when it comes to designing an actual terminal device, and connecting it to a real live exchange line - then you need to know the specifics.

I have seen too many books which purport to discuss principles but actually address the specifics of only 1 particular system (commonly the US system)

For example:

1) Every telephone line in the world is is powered with DC

What voltage? How much current? Is the polarity known, or significant?

2) Every telephone exchange senses the OFF-HOOK condition by measuring the loop current.

What current must you "pull" to make the exchange "see" your off-hook?

3) Every telephone in the world rings when AC signal of sufficient amplitude and certain frequency is applied to the line.

What frequency? What waveform? amplitude? is it pure AC, or with a DC bias?
In the UK, a capacitor is used to separate the ring signal on internal wiring.

etc, etc - these (and many more) are the details you need to know to actually design a terminal device; and these need to be the right details for your territory for it to not only work, but be guaranteed to work - and be acceptable to the authorities.