What is the public switched telephone network?
The public switched telephone network (PSTN) is the traditional phone network: the copper lines, telephone exchanges, undersea cables and carrier interconnections that let any landline call any other landline in the world. It’s sometimes called POTS, short for “plain old telephone service”.
The PSTN is “switched” because it uses circuit switching. When you make a call, the network sets up a dedicated path between the two phones and holds it open for the whole conversation. That design made the PSTN very reliable and predictable, but also inflexible and relatively expensive to run.
How the PSTN works
In its classic form, the PSTN has a few layers:
- The local loop. A pair of copper wires connects a home or business to the local exchange.
- Exchanges. Switches at the exchange route the call, either locally or onward through the network.
- The core network. Carriers link exchanges together, and connect to other carriers and countries.
- Numbering. A national numbering plan assigns area codes, mobile prefixes and special ranges such as freephone numbers.
Over the decades much of the core moved to digital and then IP technology, but the copper connection to premises and the circuit-switched model remained for many customers.
PSTN versus VoIP
Modern voice increasingly runs as Voice over IP (VoIP), where calls are sent as data packets. For businesses, SIP trunking has largely replaced physical PSTN lines as the way to connect a phone system to carriers. The PSTN still matters as the shared network that makes every number reachable, but the copper lines and old exchanges behind it are being replaced.
The practical differences for organisations:
- Flexibility. IP voice can be scaled, moved and redirected in software. PSTN lines are tied to a physical location.
- Cost. IP services are generally cheaper to provide and maintain.
- Power dependence. Traditional copper phones often kept working in a power cut because the exchange supplied power down the line. IP-based phones need power at the premises, so battery backup becomes important.
The NZ and Australian context
New Zealand’s copper network and PSTN are being retired in favour of fibre and IP-based services. Australia has been through a similar transition, with voice services moving onto its national broadband network. In both countries, organisations need to check anything still relying on copper lines, such as monitored alarms, lift phones, medical alarms and some EFTPOS or fax equipment.
For utilities this has an extra dimension. When a power cut affects homes that rely on fibre or other IP-based phones, those customers may not be able to call from a landline without battery backup, and will reach for their mobiles instead. That is one reason outage contact plans need to cover mobile-friendly channels and proactive outage notification, and why medically dependent consumers need an emergency plan that doesn’t depend on one phone line.
Why it matters for contact centres
Most contact centres now connect to the phone network through cloud platforms such as CCaaS, so the PSTN retirement rarely affects the contact centre itself. What it changes is how customers call, and how resilient their connection is during an emergency.