Pressing the keys on a telephone sends signals through wires to a local telephone exchange. A numbering plan stored in a computer at the exchange tells the exchange when a complete number has been dialled. If the phone you are calling belongs to a different exchange, your exchange sends signals to other exchanges to set up a route for your call.

Most calls from fixed phones travel to the local exchange through copper wires. Each phone has its own line card – a circuit that is permanently connected to the phone. This responds with a dialling tone when you pick up the phone. It also converts your call into electrical pulses, so that it can be handled by computers that route the call.

Nearly all calls between big cities now travel as laser light through thin glass fibres, called optical fibres. The laser switches rapidly on and off to send out high-speed digital codes. Clever coding squeezes as many different calls as possible into each optical fibre, but allows them to be sorted out again when they arrive at the next telephone exchange.

Some calls, particularly those to isolated areas, make part of their journey by riding on a beam of microwaves. These very short waves are focused by a dish-shaped reflector on a tower and sent from point to point in a straight line. Microwave links are quick and cheap to set up, as there is no need to dig tunnels or erect poles to carry fibres or wires.

Eventually the call reaches the local exchange that handles the telephone you have dialled. There, it is directed to that phone’s line card and the signal is changed back to analogue form. A pulsing current sent down the line rings the phone. When the phone is picked up, a switch in the receiver completes a circuit that cuts off the ringing current and connects the call.

Optical fibre glass is so pure than you could see through a mile of it. It is even more transparent to the invisible laser light that it carries. The inner core is covered with a layer of less heavy glass, and the light is reflected (and so trapped) where the two kinds of glass meet. A plastic coating on the outside makes the fibre tougher and easier to handle.

Delicate optical fibres are heavily protected when laid on the sea bed. Each cable contains several fibres. Some may not be needed at first but these “dark fibres” will be brought into use when calls on the cable route increase.