Why does a landline phone work without electricity?

Published on
November 17, 2021
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Communication and access to emergency services is often taken for granted. Many don't know how these services are kept alive during power outages. Does a landline phone work without electricity?

Most of the UK's population (95%) has at least one mobile phone which they can use during a power cut. However, for the 1.1m UK voice-only landline customers, landline phones are their only connection to the outside world (OFCOM).

For now, despite landline phones being seen as old technology, corded/wired landline phones will continue to work during a power outage. This is because they are powered by local telephone exchanges, which have backup power available (i.e. power separate from the mains).

This blog will explain why does a landline phone work without electricity in detail, what Voltage do UK telephone lines take and its implications on the UK's energy consumption. We will also touch on how UK copper switch off will impact landline phones post-2025.

Why does a landline phone work without electricity, and do telephone lines carry electricity?

OFCOM (the UK telecommunication industry's regulatory body) requires telephone service providers to ensure all customers can call the emergency services at any time, including during a power cut.

To provide these services, telephone lines must carry electricity separately to the mains and traditional power lines.

This requirement is enforced through the USO (Universal Service Obligation). The USO requires service providers to ensure methods of communication are safeguarded against the loss of power. In many cases, this power comes from telephone exchanges (covered in more detail below) to your home through a copper wire powered separately from the mains.

Other than landline phones, many other devices also work during a power cut. These devices are classed as critical infrastructure - What else is powered by telephone exchange lines?

  • Emergency lift alarms
  • Cash machines
  • Emergency doors
  • House alarms
  • Payment terminals

Much more relies on telephone exchanges than many people know. Without them, vulnerable customers dependent on backup power through the PSTN (Public Switched Telephone Network) could be at risk.

How does a landline phone work without electricity?

Traditional wired landline telephones are connected to local telephone exchanges via the PSTN. The PSTN provides infrastructure and services for public telecommunication, including a backup telephone line power supply.

There is often an emergency generator within telephone exchanges that kicks in during a power cut to provide electricity to critical infrastructure. These generators have enough fuel for seven days and are often backed up by batteries. These batteries are trickle fed every day to keep them topped up with nine days' worth of power, providing an additional runway for the exchanges until the mains is restored.

These batteries are required in case the generator doesn't start when it's needed. Given some equipment within exchanges is up to 30 years old, it's not surprising that things don't always work when called upon!

Once the backup power kicks in, the electricity will flow from the telephone exchange to your home and landline telephone using the same copper cabling that transmits voice calls.

Telephone Line Voltage UK - What Voltage are telephone lines?

The Voltage supplied to a landline telephone varies depending upon many factors. These factors include the country in question and the distance away from the exchange (copper wires have a resistance of ~1000ohms). The telephone line voltage in the UK, at the subscriber's end, is:

  • On-hook Voltage is 48 Volts DC
  • Off-hook Voltage is 3-9V DC
  • Phone ring voltage 90-130V AC at 20 Hz (which is superimposed on top of the on-hook 48 dc voltage)

On and off-hook are the two states of a communications circuit - i.e. the phone is either on the hookswitch/" phone holder" (such as below) or off.

Why does a landline phone work without electricity? Telephone Line Voltage UK
Landline telephone "on-hook"

In the US, the on-hook Voltage of landline telephones is slightly different to the UK's, at 50V. Furthermore, phone lines are more commonly carried on utility poles rather than under the ground.

Telephone Line Power Usage - How much energy does the PSTN consume?

To provide services through the PSTN, a large amount of electricity is consumed.

This electricity gets consumed during service provision and lost during transmission between telephone exchanges and premises. Energy is lost because copper wires have resistance, meaning the current drops over distance as electric energy is converted into heat.

Such high amounts of electricity are consumed during the provision of services that the PSTN is responsible for up to 0.7% of the UK's energy consumption (source).

UK Copper Switch-off

By December of 2025, copper infrastructure (the PSTN) in the UK will be switched off. This switch-off means all PSTN-dependant products and services will have to be migrated to next-generation alternatives. Openreach estimate that this will require around 16 million lines and channels to be migrated between now and 2025.

Significant implications that will follow on from copper switch off include:

  • Critical infrastructure will have to be carefully switched over by telephone companies without impacting service quality and reliability.
  • Backup power will no longer be provided by the PSTN and instead will have to come through local batteries in the customers' premises. This electricity will come through standard plugs and sockets, internal wiring and modern phone sockets, as even cordless phones require electricity.
  • Service providers will encourage voluntary migration as much as possible before forced migration occurs closer to the deadline.

To learn more about the UK copper switch-off and Digital Upgrade, check out our previous blog, The Complete Guide to the UK's Digital Upgrade.

Landline/Corded Phones During Power Cuts FAQ

Q: Do landline phones need batteries?

Landline phones connected to the PSTN don't need batteries; however, if you don't have a corded landline phone, you likely need a battery backup to ensure it still works during a power cut.

Q: How do landlines get power?

Landline phones are powered by copper cabling connected to the PSTN (Public Switched Telephone Network). The PSTN has backup power, which comes from generators and batteries located in telephone exchanges.

Q: Does a landline phone die when the power goes out?

No, if the power goes out in your home, your landline phone will not die when you lose electricity. Your telephone wiring is connected to copper wire powered separately to the mains.

Q: Do I still need a landline phone?

While a landline phone is a reliable way to make telephone calls during an emergency, the UK's digital upgrade means your copper phone line will be phased out in the next few years. Therefore it may be best to switch to a more modern alternative.

Q: Why do landline phones work during a power outage?

Landline phones work in a power outage because of OFCOM's USO (Universal Service Obligation). The USO requires service providers to ensure methods of communication are safeguarded against the loss of power.

Power Supply From Telephone Lines Summary

We hope you understand much more about does a landline phone work without electricity, Telephone Line Voltage UK, telephone line power supply and the implications of PSTN switch off.

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