The dial tone was introduced when we transitioned from human switchboard operators to automated telephone switching exchanges on the public switched telephone network (PSTN), more than a hundred years ago. Its purpose was to indicate to the phone user that the exchange was ready to accept the number, hence the name ‘dial tone’.
Cellular telephone services, which don’t use the PSTN, don’t generate dial tones. Our mobile phones wait for us to type the entire number before connecting and transmitting it at once. Your mobile might imitate a dial tone to make you feel comfortable, but it’s not performing the function it does for a landline phone.
Over the years, the PSTN has grown to connect the world, with wired connections everywhere, but it’s now unsuited to handle voice, video and data. To help keep things more manageable and future-proof, the UK is switching off its PSTN in December 2025, with all services switching to the internet. This is the biggest change to our phone network for 100 years.
The dial tone is generated by the PSTN exchange, which provides power to your phone and detects when you lift the receiver. When the PSTN is switched off, power will no longer be provided and your devices will have to rely on their own local power supply, so you may need a new landline phone.
This also has major implications for many kinds of device, for example: health monitors, secure phones (BRENT phones), building management systems and emergency phones in lifts, all of which need to be modified so that they continue to work.
Once we have only the internet-based system, the dial tones will be largely redundant. It’s likely that we’ll continue to hear them as many services use a dial tone to provide information, such as the system is operational; or a pulsed tone to indicate you have voice mail. But in reality, the dial tone will no longer be required and could be phased out altogether.
Read more:
- Who really invented the telephone?
- How does a smartphone ‘read’ my fingerprint?
- Is there any point turning my phone to ‘flight mode’ on a plane?
- How does the phone network know where to send the signal when someone rings me?
Asked by: Suleman Said, via email
To submit your questions email us at questions@sciencefocus.com (don't forget to include your name and location)
