How did landline phones work during power cuts?

Red vintage telephone isolated on white background. | Photo Credit: Krittiraj Adchasai The secret lay far away from your home. Unlike today’s smartphones, traditional landline phones did not depend on the electricity coming into your house. Instead, they drew power through the same copper wires that carried your voice. These wires were connected to a telephone exchange, a central hub that linked thousands of telephones together.The exchange constantly supplied a small amount of direct current (DC) electricity, usually around 48 volts, through the telephone lines. A traditional landline phone needed very little power, just enough to carry your voice and ring when someone called. Because the electricity came from the exchange rather than your home’s electrical supply, the phone continued to work even if your neighbourhood lost power.When you spoke into the handset, a small microphone converted your voice into electrical signals. These signals travelled through the copper telephone lines to the exchange, which routed them to the other person’s phone. There, a speaker converted the signals back into sound.In other words, while your lights, television and refrigerator stopped working during a blackout, the telephone line still carried both your conversation and the electricity needed to keep the phone alive. That clever design made landlines one of the most reliable ways to stay connected during emergencies.What is a telephone exchange?Think of a telephone exchange as the brain of the landline network. It is a central building where thousands of telephone lines from homes, offices and businesses come together. Whenever you dialled a number, the exchange recognised where the call needed to go and connected your phone line to the other person’s. In the early days, these connections were made manually by operators using switchboards. Later, automatic electronic systems took over, allowing calls to be connected within seconds.But the exchange did more than just connect calls. It also supplied a steady stream of electricity through the copper telephone lines. Since every landline was linked directly to the exchange, each phone received the small amount of power it needed to function. This clever setup meant that even if the electricity in your home went out, the exchange could keep your landline running. A view of the last piece of manual telephone exchange at Doddaballapur, near Bangalore which was eliminated in the State. PHOTO: THE HINDU ARCHIVES But what if the telephone exchange lost power?You might wonder: if the exchange supplied electricity to every landline, what happened if it had a power cut?Telephone networks were designed for such situations. Telephone exchanges were built with large banks of backup batteries that could immediately take over if the main electricity supply failed. If the outage lasted longer, diesel generators automatically switched on to keep the exchange running. Together, these backup systems kept telephone services running for hours, or even days, during major power failures. Color illustration representing the phone evolution. | Photo Credit: Getty Images This is one reason traditional landline phones were considered so reliable. During storms, floods, earthquakes and other emergencies, they often remained one of the few ways people could contact family members or call emergency services. Here’s a fun example from our own newsroom. During the heavy rains that lashed Chennai in 2023, mobile networks became unreliable in some areas. The Teen Digest’s editor had to rely on a traditional landline to contact the office and share work updates. At a time when modern communication struggled, the old landline turned out to be an unexpected lifesaver.Fun factThe first practical telephone was patented by Alexander Graham Bell in 1876. At first, there were no dials, people had to ask a switchboard operator to connect their calls. In 1892, American undertaker Almon Brown Strowger invented the automatic telephone exchange, allowing callers to connect without an operator. His invention paved the way for the familiar rotary dial phones, which became popular in the early 1900s. To make a call, you placed your finger in numbered holes on a circular dial and rotated it for each digit.Why doesn’t this happen today?Many modern home phones no longer use traditional copper telephone lines. Instead, they work through fibre broadband or the internet (VoIP). That means they depend on equipment such as routers and modems, which need electricity. So, if there’s a power cut, these phones usually stop working too, unless you have a backup power source. Published - July 27, 2026 02:39 pm IST