TL;DR
PSTN, or Public Switched Telephone Network, is the global system of interconnected telephone networks that carries traditional voice calls. CPaaS and cloud-voice platforms connect applications to the PSTN so businesses can call ordinary mobile and landline numbers through APIs, SIP, or programmable voice systems.
At a glance
Question | Answer |
|---|---|
What does PSTN stand for? | Public Switched Telephone Network |
What does it do? | Connects telephone numbers and carries voice calls |
Is PSTN the same as VoIP? | No. PSTN is the traditional telephone network; VoIP sends voice over IP |
How does CPaaS use PSTN? | It connects programmable voice applications to ordinary phone numbers |
Does PSTN still matter? | Yes. Most customers still use mobile or landline numbers reachable through carrier networks |
What does PSTN connect? | Mobile networks, landlines, carriers, PBXs, voice platforms, and telephone numbers |
What is PSTN?
PSTN stands for Public Switched Telephone Network. It is the interconnected system of telephone networks used to make voice calls between telephone numbers.
PSTN includes:
- Public telephone networks
- Mobile carrier networks
- Fixed-line networks
- Numbering systems
- Switching infrastructure
- Carrier interconnections
- International termination routes
- Voice-service providers
Although modern communications increasingly use IP networks, the PSTN remains important because customers still use ordinary mobile and landline numbers.
A business using a cloud voice platform may have a browser-based application on one side and an ordinary customer phone number on the other. PSTN connectivity allows those two sides to communicate.
How does PSTN calling work?
A simplified call path looks like this:
For an inbound call, the direction is reversed:
The call may pass through multiple carrier networks depending on the location of the caller, recipient, and business.
PSTN versus VoIP
These terms are often confused.
PSTN
PSTN is the traditional public telephone infrastructure used to connect calls between telephone numbers.
VoIP
VoIP, or Voice over Internet Protocol, is a method of transmitting voice over an IP network.
A VoIP application can connect to:
- Another VoIP application
- A SIP endpoint
- A cloud PBX
- A CPaaS platform
- The PSTN
The two systems can work together. A business may use VoIP inside its software while relying on PSTN connectivity to call customers on ordinary phone numbers.
PSTN versus SIP trunking
SIP trunking is a method of connecting a business voice system to a provider or carrier through IP.
PSTN is the public telephone network that ultimately connects many calls to mobile and landline numbers.
A common architecture is:
SIP trunking and PSTN therefore solve different parts of the voice problem.
Read the SIP trunking glossary entry for more information about the IP-based connection between business systems and voice providers.
How CPaaS uses PSTN
CPaaS platforms use PSTN connectivity to provide programmable communication features such as:
- Inbound voice calls
- Outbound voice calls
- Virtual phone numbers
- IVR
- Call routing
- Call transfer
- Call recording
- Voice notifications
- AI voice agents
- Conference calls
- Click-to-call
- Call masking
- Number verification
A developer may use a Voice API to instruct a CPaaS platform to call a customer. The platform handles the voice infrastructure and connects the call through the relevant carrier or PSTN route.
Helo.ai’s What Is CPaaS guide explains how programmable communication platforms connect applications with voice, messaging, and other channels.
PSTN and phone numbers
PSTN communication depends on numbering systems. A business may use:
- Local numbers
- National numbers
- Mobile numbers
- Toll-free numbers
- DID numbers
- Emergency-service numbers
- International numbers
The availability and use of each number type varies by country.
A DID number allows an inbound caller to reach a particular business, application, department, or workflow. For example, a CPaaS platform may route one number to a support queue and another number to a sales team.
Review the DID number glossary entry for more information.
PSTN call origination and termination
Call origination
Call origination refers to the point where a call enters the provider or communication network from the caller.
For example:
text
Customer mobile → carrier → PSTN → business CPaaS platform
Call termination
Call termination refers to the final network path that delivers a call to the recipient.
For example:
text
Business CPaaS platform → carrier → PSTN → customer mobile
A provider may have different rates, routes, quality, and regulations for origination and termination.
PSTN and call quality
PSTN call quality can be affected by:
- Carrier routing
- Network congestion
- Geographic distance
- Interconnection quality
- Codec conversion
- IP connection quality
- Mobile coverage
- Device quality
- Call setup delay
- Packet loss on the IP side
A cloud platform may provide a high-quality application interface, but the final call can still be affected by the recipient’s mobile network or location.
Businesses should monitor:
- Answer rate
- Failed-call rate
- Call setup time
- Post-dial delay
- Dropped calls
- One-way audio
- Call duration
- Transfer success
- Customer-reported quality
Helo.ai’s intelligent call-routing guide provides practical context on routing calls based on availability, intent, and business rules.
PSTN and CPaaS compliance
Businesses using PSTN connectivity may need to manage:
- Number ownership
- Caller-ID rules
- Call recording disclosures
- Consent for outbound calls
- Do-not-call requirements
- Robocalling restrictions
- Emergency calling
- Data protection
- Local telecom regulations
- Number registration
Rules vary significantly across countries and use cases. A system designed for India may not follow the same requirements as a system designed for the United States, United Kingdom, or European Union.
The provider should explain:
- Which numbers are available
- How numbers are verified
- What caller ID is displayed
- Whether international calls are supported
- How emergency calls are handled
- How call recordings are stored
- How long call metadata is retained
India-specific PSTN considerations
Indian businesses using programmable voice should evaluate:
- Local number availability
- Domestic calling routes
- Caller-ID requirements
- TRAI regulations
- Outbound-calling consent
- Recording and disclosure requirements
- DND preferences
- Call timing restrictions
- Telecom provider responsibilities
- Multilingual IVR and voice support
For example, a customer-service operation serving users across Mumbai, Delhi, Bengaluru, and smaller cities may need to combine PSTN calling with multilingual IVR, local routing, and AI voice support.
Helo.ai’s Best IVR Service Provider in India provides additional context on multilingual voice, call routing, and India-focused compliance considerations.
PSTN versus mobile and internet messaging
PSTN is designed primarily for telephone calls. It is different from:
- SMS
- RCS
- WebRTC-only communication
- App-based chat
A CPaaS platform may combine PSTN voice with these channels to create an omnichannel customer journey.
For example:
- An SMS sends a payment reminder.
- The customer replies through WhatsApp.
- A voice bot calls if the issue requires immediate attention.
- A human agent takes over through PSTN.
- The interaction is recorded in the CRM.
Benefits of PSTN connectivity through CPaaS
- Connects applications to ordinary phone numbers.
- Enables inbound and outbound business calls.
- Supports programmable voice workflows.
- Provides access to carrier networks.
- Enables IVR and call routing.
- Supports virtual business numbers.
- Connects remote agents and cloud systems.
- Allows voice to work alongside SMS, WhatsApp, RCS, and email.
- Supports AI voice agents and automated notifications.
Limitations and considerations
- Telecom regulations vary by country.
- Number availability is not universal.
- Call quality depends on carriers and networks.
- International termination can be expensive.
- Emergency calling may require separate arrangements.
- Caller-ID rules can restrict display options.
- Recording may require consent and disclosure.
- Poor IP connectivity can affect cloud-side audio.
- Carrier routing may change.
- PSTN calls are not end-to-end encrypted in the same way as some app-based messaging services.
Frequently asked questions
Is PSTN still used today?
Yes. PSTN and connected carrier networks remain important because customers continue to use mobile and landline telephone numbers.
Is PSTN the same as a landline?
No. PSTN includes the broader public telephone infrastructure used for fixed-line and mobile voice communication.
Can CPaaS connect to PSTN?
Yes. CPaaS providers can connect programmable applications to ordinary telephone numbers through carrier and PSTN connectivity.
Can a WebRTC call reach a PSTN number?
Yes, if the WebRTC application is connected to a CPaaS platform or voice provider with PSTN access.
Is PSTN secure?
Security depends on the network, provider, application, signalling, media, recording, and access controls. PSTN calls should not automatically be described as end-to-end encrypted.
Does PSTN support caller ID?
Caller-ID presentation depends on the country, carrier, number ownership, provider, and applicable rules.
Does PSTN support call recording?
A CPaaS or contact-centre platform may support recording, but recording requires appropriate notification, consent, storage, retention, and access controls.
What is the difference between PSTN and SIP?
PSTN is the public telephone network. SIP is a protocol used to establish and manage communication sessions, including calls that connect to the PSTN.