The Public Switched Telephone Network (PSTN) is the interconnected public network of telephone carriers, switches and interconnections that carries calls between routable phone numbers, whether the call starts on a landline, a mobile phone or a VoIP or cloud phone system. It was built as a circuit-switched network, where each call reserved a dedicated path for its duration. Carriers have since moved much of it to IP technology, but “the PSTN” still means the shared public system of phone numbers and carriers that business phone systems connect to.
At a glance
- The PSTN is the global public phone network: local, long-distance, mobile and international carriers interconnected under shared numbering.
- Phone numbers follow an international numbering plan (E.164), with country codes and national numbering rules.
- Business phone systems, including cloud and Teams-based ones, need “PSTN connectivity” to call outside numbers.
- Connections to the PSTN range from analog copper (POTS) and PRI circuits to SIP trunks and cloud calling plans.
- Emergency calling, number porting and caller ID all depend on how you connect to the PSTN.
What problem it solves
The PSTN is what makes a phone number broadly reachable. Because carriers interconnect and follow common numbering and signaling standards, a caller usually does not need to know which carrier the other party uses. Reachability is not absolute: it can depend on the number type (toll-free, premium or short codes), the carriers involved, country restrictions, routing arrangements, and fraud controls or call blocking. It also carries emergency calls to public safety answering points in most countries.
For a business, the practical question is not whether to use the PSTN but how to connect to it. Internal calls between employees can stay entirely inside a cloud phone or collaboration platform, but calls to customers, suppliers and emergency services still go through the PSTN.
How it works
Historically, local exchanges (central offices) connected subscriber lines and switched calls through tandem and long-distance switches, reserving a circuit for each call and using a separate signaling network (SS7) to set calls up and tear them down. Mobile networks and VoIP providers interconnect with that system so their customers can reach traditional numbers.
Today much of the network’s core and many carrier interconnections run over IP, often using SIP, the same signaling protocol used by business phone systems. The pace of that transition varies by carrier and country.
Businesses connect to the PSTN in several ways:
- Analog lines (POTS) for single phones, fax, alarms and other simple devices; these are being retired in many areas (see POTS replacement).
- PRI circuits that carry 23 or 30 calls to an on-site PBX.
- SIP trunks that carry calls over IP to a PBX or session border controller.
- Cloud calling included with a hosted PBX, UCaaS or contact center service, or connected through options such as Microsoft Teams Direct Routing.
Whichever method you use, the carrier assigns or hosts your phone numbers, routes emergency calls with location information where required (see E911) and passes caller ID. In the US and Canada, voice providers also authenticate caller ID on IP-based calls using STIR/SHAKEN, a framework regulators have phased in since 2021; the attestation level your calls receive is one of the factors that can affect whether they are labeled as spam.
When it matters for buyers
- When moving to a cloud phone system or Teams, because PSTN connectivity is often licensed and priced separately from the platform.
- When copper or PRI services are being retired or repriced, forcing a change in how each site connects.
- When expanding internationally, since number availability, regulations and calling rates differ by country.
- When outbound calls are flagged as spam, which can relate to caller ID authentication and number reputation.
- When reviewing resilience, since the carrier connection is often the single point of failure for inbound calling.
Questions to ask vendors
- How does your service connect to the PSTN, and in which countries can you provide numbers?
- Can you port all of our existing numbers, including toll-free and DID ranges, and how long will it take?
- How do you route emergency calls, and how is each caller’s location kept current?
- What attestation level do our outbound calls receive, and how do you help prevent spam labeling?
- What are the rates for local, long-distance, international, mobile and toll-free calls, and what fees and taxes apply?
- What redundancy protects inbound calling if your network or our connection fails?
How it differs from POTS
POTS (plain old telephone service) is the traditional analog copper line from the phone company to a single phone or device. It is one way to connect to the PSTN, not the network itself. Replacing POTS lines with cellular or IP services changes the connection, not the fact that the device connects to the public network; test that the numbers and devices behind it still work as expected.
To connect Teams or another cloud phone system to the PSTN, see our Microsoft Teams PSTN connectivity solution page.
Problems this helps with
- Making a cloud phone system call regular phone numbers
- Choosing how our phone system connects to outside callers
- Keeping emergency calling working after we leave copper lines
