What is the PSTN?

The PSTN (Public Switched Telephone Network) is the traditional, circuit-switched telephone system that has carried voice calls since the late 19th century. It is the global network of copper lines, fiber trunks, switching centers, and undersea cables that connects landline and legacy phone calls. When a call is placed over the PSTN, the network opens a dedicated circuit between the two parties for the duration of the conversation. Modern business communications increasingly replace the PSTN with internet-based VoIP.

How the PSTN works

The PSTN is built on circuit switching: for each call, the network reserves an end-to-end path and holds it open until someone hangs up. That dedicated circuit guarantees consistent voice quality but uses network capacity inefficiently — the line stays reserved even during silence.

Key characteristics:

• Circuit-switched, not packet-switched — one call occupies one dedicated path.
• Standardized numbering through the national and international numbering plans, including direct-inward-dial (DID) numbers.
• High reliability and line power, which historically kept landlines working during outages.
• Fixed capacity — adding lines means adding physical or leased circuits.

PSTN vs. VoIP

VoIP converts voice into data packets and sends them over the internet, while the PSTN holds an open circuit for each call. The practical differences:

• Cost: VoIP avoids per-circuit line rental and dramatically lowers long-distance and international rates.
• Scalability: VoIP adds lines in software; the PSTN requires physical provisioning.
• Features: VoIP bundles voicemail-to-email, call recording, auto attendants, and CRM integration that the bare PSTN cannot offer.
• Resilience: traditional PSTN lines carry their own power; VoIP depends on internet and local power, mitigated with mobile failover.

The PSTN switch-off

Carriers worldwide are retiring PSTN and ISDN infrastructure in favor of all-IP networks. In several markets, legacy copper and ISDN services are being withdrawn, pushing businesses to migrate to VoIP and SIP trunking before their old lines stop working. Organizations still running analog lines, fax machines, alarm panels, or elevator phones on the PSTN need a migration plan to IP-based equivalents.

How businesses connect to the PSTN today

Even VoIP calls often need to reach a regular phone number on the PSTN. That bridge is handled by SIP trunking and carrier interconnects, which pass calls between the IP network and the remaining PSTN. A cloud provider manages those interconnects so a business can keep its numbers and still call any landline or mobile.

PSTN frequently asked questions

What is the difference between the PSTN and VoIP?

The PSTN is the legacy circuit-switched phone network that reserves a dedicated line per call. VoIP carries calls as data packets over the internet and shares network capacity. VoIP is cheaper, software-scalable, and feature-rich; the PSTN is being phased out in favor of all-IP networks.

Is the PSTN being shut down?

Yes. Telecom carriers in many countries are withdrawing legacy PSTN and ISDN services as part of a move to all-IP infrastructure. Businesses relying on analog lines are being migrated to VoIP and SIP-based services, and old copper-line products are being discontinued on published timelines.

Do VoIP calls still use the PSTN?

Sometimes. A call between two VoIP users stays entirely on IP networks. But when a VoIP user calls a traditional landline or a number still served by legacy infrastructure, the call crosses into the PSTN through carrier interconnects, typically via SIP trunking.

What equipment still depends on the PSTN?

Analog devices such as older fax machines, elevator emergency phones, alarm and security panels, and some point-of-sale terminals were built for PSTN lines. These need IP-based replacements or analog-telephone-adapter workarounds before a PSTN switch-off removes their connectivity.

See how DialPhone replaces the PSTN

DialPhone runs entirely on a modern IP network, so businesses get cloud calling with local numbers, free porting, and built-in AI features — without leasing PSTN circuits or worrying about copper-line retirement.