Hello Andy,

you wrote:
But whose telephone set and whose telephone line

Please have a look at following points

1) Every telephone line in the world is is powered with DC
2) Every telephone exchange senses the OFF-HOOK condition by measuring the loop current.
3) Every telephone in the world rings when AC signal of sufficient amplitude and certain frequency is applied to the line.
4) Every telephone in the world uses either DTMF or PULSE dialling.
5) Every telephone in the world has some 2 wire to 4 wire conversion circuit inside it.
6) Every telephone has a speaker and a microphone.
7) Every telephone has some sort of hook switch which disconnects the speech circuit from telephone line when pressed.

And lots more, but same principles for every telephone of the world

Can you quote me any single example of a telephone exchange/central office, which does not operate as I stated above???? ( Now please don't quote me mobile phones, which don't have any physical line, I already know them)


Remember,
A text book is not a regulatory documnet, it just gives you the basic knowledge and it may provide the examples which are related to a certain system or region, but the basic principles of telephony have been same for all the telephones of world.

Very best regards,

IA