What is PSTN, and How Does It Work?

What is PSTN? A Guide to Public Switched Telephone Network

Table of contents

The Public Switched Telephone Network (PSTN) changed how we communicate since Alexander Graham Bell founded the American Bell Telephone Company in 1875. Initially, operators manually connected calls, but automation improved efficiency in the early 20th century.

Today, PSTN remains essential for many businesses and homes, providing reliable voice communication through a network of physical lines and switches. Despite the modern communication solutions like VoIP, PSTN continues to support areas with limited internet access.

What is PSTN?

The Public Switched Telephone Network (PSTN) is a traditional telephone system that uses a network of physical wires, cables, and switching centers for voice communication. It connects calls through a series of switches at central offices, allowing users to make local, long-distance, and international calls. Also known as landlines, Plain Old Telephone Service (POTS), or fixed lines, PSTN operates on analog and digital signals. It has been in use since the late 1800s.

History of PSTN

The history of PSTN began in 1875 with Alexander Graham Bell’s invention of the telephone and the creation of the American Bell Telephone Company. Initially, operators manually connected calls. In the early 20th century, automated telephone exchanges replaced manual systems, making the process faster and more efficient.

The transition from analog to digital technology further enhanced call quality and capacity. The introduction of fiber-optic cables in the late 20th century provided even greater clarity and bandwidth.

Modern PSTN infrastructure integrates with these newer technologies, such as a VoIP phone system, to provide traditional reliability and modern convenience for seamless connectivity.

How Does PSTN Work?

The Public Switched Telephone Network (PSTN) connects telephone calls through physical and digital components. It uses a combination of analog and digital signals to transmit voice data. Analog signals are converted to digital signals for transmission over long distances and then converted back to analog signals for delivery to the recipient’s phone.

1. Dialing a Number

When you dial a phone number, your telephone sends a signal to the local exchange, the Central Office (CO). The signal includes the digits of the number you are calling.

2. Local Exchange

The local exchange is the initial touchpoint in the PSTN. It receives the dialed digits and determines the best route for your call. The local exchange connects your call directly to the recipient’s phone line if you are making a local call.

3. Tandem Office

If you are making a long-distance call, the local exchange routes your call to a tandem office. The tandem office directs your call to the appropriate long-distance network.

4. Toll Office

The tandem office routes your call to a toll office for long-distance or international calls. The toll office manages the routing of calls over long distances.

5. International Gateway

For international calls, the toll office directs your call to an international gateway. It handles the transition of your call from the domestic network to the international network, connecting you with recipients in other countries.

6. Carrier Access Points

Carrier access points are checkpoints within the network that manage the flow of data and prevent network congestion. They ensure your call is routed smoothly, even if the network is busy.

7. Trunk Lines

Trunk lines are high-capacity communication lines connecting various PSTN components, such as local exchanges, tandem offices, and toll offices. These lines can carry many calls simultaneously, ensuring efficient communication.

8. Customer Premises Equipment (CPE)

Customer premises equipment refers to the devices used by end-users to make and receive calls. This includes telephones, fax machines, and modems. The call starts and ends with these devices.

9. Call Completion

Once the call reaches the recipient's local exchange, it is routed to the recipient's phone line, and it rings. When the recipient answers, the call is connected, and you can communicate.

How Much Does PSTN Phone Line Cost?

The cost to connect to your desired recipient through PSTN depends on whether you use it for personal or business use.

When setting up a PSTN network at home, you need to install phone lines and purchase a telephone if you already don’t have one. Typically, you pay $50 to $200 for installation. Additionally, you would also spend on monthly service fees. The services are generally available with a basic monthly fee for local calls and additional charges for long-distance or international calls.

Meanwhile, when you use PSTN for your business communication, you must set up a PBX phone system with multiple landlines. Besides the installation charges, the monthly cost for a PSTN service for businesses can range from $1000 to $5000, depending on the number of users and the features you use.

Calilio offers VoIP phone services, which eliminates the need for expensive physical infrastructure and allows you to make and receive calls over the Internet. Our cloud-based business phone system reduces installation and maintenance expenses, as there’s no need for dedicated phone lines or PBX hardware. Our  VoIP services come with a lower subscription fee, starting from $12 per month.

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Alternatives of PSTN

With the advancement of communication technologies, several modern alternatives to PSTN have emerged: ISDN and VoIP. ISDN allows for faster digital transmission of voice and data while VoIP uses internet connectivity to make and receive calls.


ISDN (Integrated Services Digital Network) is a telecommunications network that enables the digital transmission of voice and data over traditional PSTN networks. It is suitable for businesses that already have a PSTN network but are looking to increase data transmission speed with added voice clarity.

ISDN allows for multiple digital channels - you can use one line for multiple simultaneous transmissions, such as voice, video, and data. However, it requires a more complex setup and higher initial costs compared to PSTN.







Data Speed



Voice Quality



Network Configuration


More complex but versatile


VoIP (Voice over Internet Protocol) is a technology that allows voice communication over the Internet. Unlike PSTN, which relies on physical phone lines, VoIP uses your existing internet connection, reducing infrastructure costs.

It allows you to make calls on various devices like computers, smartphones, and tablets. VoIP typically has lower call rates, especially for international calls, making it a cost-effective solution.








Basic features for phone calls

Extensive business communication features


Fixed location

Accessible from anywhere with an internet connection


Typically Higher


Switch to VoIP Today

Switching to VoIP from PSTN can greatly enhance your communication capabilities. Whether you need to scale your operations, integrate multiple communication channels, or reduce expenses, VoIP offers a superior solution for modern businesses.

With Calilio’s VoIP services, you can enjoy features like call forwarding, voicemail-to-email, video conferencing, and more. Sign up to Calilio and upgrade your communication system today.

Frequently Asked Questions

What is PSTN in simple terms?

PSTN, or Public Switched Telephone Network, is the traditional telephone system that uses physical cables and switching centers to connect calls between landline phones.

What is an example of a PSTN?

An example of PSTN is the standard home telephone service, where you connect your phone to a wall jack and make calls using copper wires that link to a network of switching centers.

Is PSTN the same as landline?

PSTN is often synonymous with landline. It refers to the traditional wired phone system that connects calls through a network of physical cables.

Does the PSTN still exist?

The PSTN still exists and is widely used, especially for landline telephones. Despite the rise of digital communication methods, it remains a key global telephony option.

What is PSTN being replaced with?

PSTN is gradually being replaced by more advanced communication technologies like VoIP (Voice over Internet Protocol) and mobile cellular networks.

What is PSTN vs VoIP?

PSTN is a traditional telephone system that uses physical phone lines, while VoIP is a modern technology that transmits voice calls over the Internet.

What is the main difference between PSTN and ISDN?

PSTN uses analog signals for voice communication, whereas ISDN uses digital transmission, allowing faster data transfer and better voice quality. ISDN also supports multiple simultaneous connections over a single line.

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