The PSTN (public switched telephone network) is the worldwide network that lets any phone number call any other. It started as copper wires and switchboards, and today it is a mix of copper, fiber, cell networks and internet links run by thousands of carriers. When a VoIP provider says calls "go out to the PSTN," it means they leave the provider's own system and reach the regular phone network.
This guide explains what the PSTN is, how a call crosses it, how phone numbers fit in, how VoIP and cell service connect to it and how it's changing.
What "switched" means
In the original network, a call worked by switching: a series of switches connected a dedicated circuit from one phone to the other for the length of the call. That circuit carried nothing else while you talked. Internet calls work differently: your voice is cut into packets that share the network with all other traffic. Most of the PSTN's core has already moved to packets, but the rules, numbers and interconnections still follow the old network's shape.
The parts of the network
- The local loop: The connection from your building to the carrier: copper, fiber, cable or a cell signal.
- Local exchanges (central offices): The carrier buildings where calls are switched. Copper lines run back to them.
- Long-distance and tandem networks: Carriers that move calls between areas and between other carriers.
- Signaling: A separate system (traditionally SS7) that sets up calls, finds where a number lives and carries caller ID, apart from the voice itself.
- Interconnection: Agreements and links that let one carrier's customers call another's. Cell carriers and VoIP providers connect here.
How phone numbers work
The US, Canada and several Caribbean countries share the North American Numbering Plan, run by the North American Numbering Plan Administrator (NANPA, checked October 19, 2026). A number has three parts:
| Part | Example | What it does |
|---|---|---|
| Area code (NPA) | 212 | A region; new codes are added as "overlays" when a region runs out |
| Exchange (NXX) | 555 | A block within the area code, historically one central office |
| Line number | 0142 | The individual line |
A number isn't tied to a wire any more. With number portability you can move it between carriers, including to a VoIP or cell provider, and the network looks up who holds it on each call. See our porting guide and US phone numbers by state and area code.
A very short history
- Operators and switchboards: Early calls were connected by hand: an operator plugged a cord between two lines.
- Automatic switches: Dialing replaced operators for most calls; mechanical, then electronic switches built the circuit as you dialed.
- Digital trunks and SS7: Voice between exchanges became digital (64 kbps per call, the same G.711 format VoIP still uses to talk to the network), and a separate signaling network set up calls.
- Mobile and number portability: Cell networks joined the PSTN, and numbers became movable between carriers.
- The IP transition: Carriers are now moving the remaining switched network onto IP, retiring copper along the way.
How a call crosses the PSTN
- 1. You dial. Your carrier reads the number and checks whether it has been ported to another carrier.
- 2. Signaling sets up the path and checks that the far end is free.
- 3. The call is handed between carriers at interconnection points, sometimes through a long-distance carrier.
- 4. The receiving carrier rings the phone: a landline, a cell or a VoIP app.
- 5. Caller ID travels with the call. For calls carried over IP, US carriers sign it under STIR/SHAKEN so the receiving carrier can check it.
How VoIP and cell calls connect to the PSTN
Cell carriers are part of the public network and interconnect with it directly. VoIP providers connect through gateways and SIP trunks: the gateway converts between internet calls and the format the carrier on the other side expects. The FCC's definition of "interconnected VoIP" is literally a service that lets users receive calls that originate on the PSTN and place calls to it (47 CFR § 9.3, checked October 19, 2026). That is also why interconnected VoIP providers carry 911 obligations. Our VoIP walkthrough shows where the gateway sits in a call.
Toll-free numbers and the PSTN
Toll-free numbers (800, 888, 877, 866, 855, 844 and 833) are part of the same network, with one difference: the business, not the caller, pays for the call. When someone dials one, the network looks the number up in a shared database to find which carrier handles it, then routes the call there. Toll-free numbers can be moved between providers like any other number. See our toll-free number glossary entry, and toll-free verification if you also want to text from one.
PSTN vs VoIP: the comparison people ask for
| Traditional PSTN line | VoIP | |
|---|---|---|
| How voice travels | Dedicated circuit (on copper, to the exchange) | Packets over the internet |
| Power | Copper lines carry their own power to corded phones | Needs local power for modem, router and phones |
| Where you answer | Wired phones at one address | Any device signed in to the account |
| Features | Basic; extras from the carrier | Software features: apps, recording, menus, integrations |
| Direction | Being retired by large carriers | Where service is moving |
In practice it isn't either-or: a VoIP call to a landline uses both, and much of the PSTN core already runs on IP.
The PSTN is turning into an all-IP network
Carriers are replacing the old switched network with IP. In March 2026 the FCC adopted an order to speed that change by simplifying how carriers retire copper and discontinue legacy services, while keeping protections for 911 continuity (FCC 26-19, adopted March 26, 2026; Chairman Carr's statement; both checked October 19, 2026). For a business, that mostly affects copper lines and the devices still on them. Our guide to POTS lines covers what to do.
What this means for your business
- Your business number can live with any carrier or VoIP provider, and move between them.
- Copper lines are being phased out; plan replacements for anything still on one.
- Caller ID trust now depends on how your provider signs your calls; a verified business number helps calls avoid spam labels.
- Text messages travel on separate messaging networks with their own registration rules, like 10DLC and toll-free verification.
How Chargezen helps
Chargezen numbers are real US phone numbers on the public network, so any customer can call or text them from any phone. Concierge answers them, and your team joins when needed.
- One US number for calls and texts: Get a toll-free or local US number in Chargezen, search open numbers by area code on phone numbers, or bring the number you already have. Chargezen files the carrier registration texting needs. See a business phone number.
- Concierge answers every call and text: Day and night, with the opening line you write, answering from the information you give it. It recognizes returning callers by their number. See the AI receptionist and the phone channel.
- Teammates join from Finn or the browser: When a customer needs a person, Concierge alerts your team in the Finn app and the browser, and emails or texts the teammates you pick. Listen in with your microphone off, tap Join the call, and Concierge tells the caller you're there. From the Finn app on iPhone or Mac you can transfer the call to a teammate, with a note. See phone calls.
- One thread per customer: Calls, texts and the rest of the customer's history sit together in the team inbox, so whoever picks up next can see what was said.
To be plain about what Chargezen is not: it doesn't sell VoIP lines, desk phones or a PBX. It has no extensions, phone menu, ring groups, call forwarding, simultaneous ring or business-hours routing, and it offers US (+1) numbers only. If you need those, buy a phone system built for them. Chargezen answers the calls and texts on your business number, and your team works the same conversations from there.
Concierge Starter is $20 a month with 20,000 credits. Pro is $199 a month with 199,000 credits, or $1,990 a year. Phone and text charges are billed as used, with no per-user fees. See pricing.
Related terms: VoIP, number porting, STIR/SHAKEN and toll-free number. Related guides: POTS lines and copper retirement, what VoIP is and how to get a business phone number.
Frequently asked questions
Is the PSTN the same as a landline?
A landline is one way to connect to the PSTN. Cell phones and VoIP services connect to it too.
Is the PSTN going away?
The copper-and-circuit version is being retired. The public network itself, where any number can call any other, continues on IP.
What is a PSTN gateway?
A device or service that converts calls between VoIP and the traditional phone network, so internet calls can reach regular numbers.
Do texts travel over the PSTN?
Texts use the mobile messaging networks, which are separate from voice. Business texts have their own registration rules; see our 10DLC guide.
What does "PSTN fallback" mean?
Some phone systems can send calls over a regular phone line or a cell network when the internet connection fails. Ask your provider whether it offers this, and whether calls can ring a mobile number during an outage.
