What is the Public Switched Telephone Network (PSTN)?

This post covers the PSTN: what is it, how does it work, and how does it differ from new voice technologies such as hosted PBX and VoIP?

pstn switch board
Ginger Woolridge

Apr 21, 2022


While Voice over IP (VoIP) has rapidly become the standard for enterprise telephony solutions, there are some modern networkers who might not understand the role that the Public Switched Telephone Network (PSTN) used to and, in some capacity, still does play in their telephony. 

Even with the most robust UCaaS solutions, the PSTN can still be involved (e.g. direct routing with Microsoft Teams). 

So today we’re covering the PSTN  - what is it, how does it work, and how does it differ from new voice technologies such as hosted PBX and VoIP?

What is the PSTN? 

The PSTN has been transporting voice communications via copper wires since the 1800s. 

The PSTN is the part of the network where calls come in and are routed or “switched” to their final destination. 

When you think of old-school telephony, where phone calls are made via landline to the operator who connects you via circuit board - that’s the OG PSTN. With the modern PSTN, there isn’t a physical operator making those switches, but rather calls are routed by their prefixed international and area codes.  

How Does the PSTN Work?

The PSTN relies on analog electronic pulses and circuit-switching to move information from place to place. Placing a call starts when a person picks up the phone and dials a number, which is converted into a sequence of analog electrical signals. A copper wire typically moves the pulses from their start to a nearby terminal. 

The terminal transmits the call to the telecommunication service provider’s Central Office (CO). There, special purpose CO voice switches route calls to their ultimate destination, which usually has a mirrored set of equipment on the other side.

Growing up you might remember how your landline telephone would still work even if the power in your house was out. This is because Plain Old Telephone Service or POTS lines are powered by the central office.  Telephone companies go to great lengths to ensure the power plant at COs is uninterrupted.  For this reason, traditional telephony is still in use today where the ability to communicate is absolutely mission critical, such as connecting fire alarms or elevator emergency lines. This will no longer be the norm, however, when the FCC mandated copper sunset kicks in (which is August 2, 2022, as of this writing). 

The PSTN system establishes a direct link between the two end points, which is a reliable but not very flexible or efficient communications transport. Additionally, PSTN technology relies mainly on hardware which requires a great deal of manual interaction during installation, maintenance, and enhancements. Both of these reasons are why the likes of VoIP and virtual PBXs have been growing in popularity. 

Alternatives to the PSTN 

Through the years, a few options have emerged so businesses could route calls in different manners than over the PSTN and a local carrier’s service. 

Hosted PBXs operate much like a private PSTN telephone network whereas VoIP networks replace the legacy equipment with modern, digital, IP network devices.

Private and Hosted PBXs 

A Private Branch Exchange (PBX) is a slimmed down version of a CO switch. The device functions in the same way but is geared to supporting a single business’ calls rather than a national voice network. In some cases, businesses deployed PBXs on their premises and managed their own voice networks.

Purchasing these devices represents a significant upfront capital expense. Setting up an on-premises PBX system network can be difficult and requires deep technical expertise. In addition, the enterprise needs to buy the equipment, set it up, and manage it. The customer also becomes responsible for all system maintenance and must upgrade the system continually. Eventually, the solution will age and need to be replaced.

An alternative is a hosted PBX service. Here, a third party telecommunications solutions provider deploys and monitors the equipment. The customer typically pays them a set monthly fee.


A VoIP call relies on a digital foundation. The caller uses a SIP (Session Initiation Protocol) handset, laptop, or smartphone to dial the number. The end point runs on a wired or a wireless internet connection to reach an access point that feeds the information into a router that moves the data (VoIP also works on LTE, satellite, fiber… etc) to the Internet. 

The network equipment features digital packet switching, which is more dynamic and efficient than circuit switching for moving Wide Area Network (WAN) traffic. VoIP doesn’t use dedicated lines. Instead, routers move data packets through the least congested and shortest path at the time of the interaction. It will take one path one moment, and switch to another if the line becomes congested.  In essence, VoIP turns your voice communications into real time streaming audio between two connected endpoints. 

VoIP has a modern architecture, one based on IP, which has become the common way to link billions of devices across the globe. Here, software has been decoupled from the underlying hardware, providing more modularity, more intelligence, and more flexibility than PSTN devices. As a result, these systems are also simpler to install and manage than PSTN equipment.

VoIP also supports high-quality sound. Digital lines often deliver a noticeable difference in call quality than analog lines, which can become muffled or fuzzy. They work with advanced features. Carriers offer customers premium features, such as auto attendants, call recording, and call blocking.

PSTN, VoIP and PBXs... oh my

The PSTN has long been a business communications staple. But recently, the PSTN has been losing favor to VoIP, which typically offers lower costs, simpler maintenance, and richer feature sets. 

We hope you found this post helpful in breaking down the PSTN as well as other technologies in the voice space that offer similar capabilities.

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