What is PSTN, and how does a call from the cloud reach a regular phone?
PSTN (Public Switched Telephone Network) is the public telephone network: all the lines, exchanges and cellular networks that connect every telephone number in the world. A cloud PBX speaks over the internet, and at a special crossover point its call enters this network — and that is how it reaches any regular phone or cell phone.
Definition and origin
PSTN stands for Public Switched Telephone Network. It is the network that was built over more than a century: copper wires to homes, telephone company exchanges, cables between cities and undersea cables between countries. Today it also includes the cellular networks and the newer telephony providers.
The word "switched" comes from the days when an operator connected two lines by hand with a cable. Later came electro-mechanical switches, and after them computers. The principle hasn't changed: when you dial, the network "connects" the two sides for the whole length of the call.
In everyday language, PSTN simply means "the network of all telephone numbers." When we say a call "went out to the PSTN," it means it left your PBX and entered the regular telephony world — to a customer's cell phone, to another office's line, or to a number abroad.
Two worlds that talk to each other
A cloud PBX works over VoIP: the voice is cut into small packets and travels over the internet, and the call is controlled with the SIP protocol. The public telephone network was built on a different principle — a dedicated channel for each call, with its own rules and standards.
For the two worlds to talk, you need a crossover point. It is called a gateway, and in the connection between a PBX and a provider, a SIP Trunk. On one side a SIP call comes in, on the other side a regular call goes out to the telephone network, and the other way around.
The simple analogy: a port. Cargo arrives by ship, and at the port it is moved onto a truck that drives on the roads. The ship and the truck don't need to know each other — the port makes the match.
The path of an outgoing call, step by step
- The secretary at the office dials from her extension to a customer's cell phone.
- The phone sends a SIP request to the cloud PBX: "I want to call this number."
- The PBX checks that the extension is allowed to dial there, chooses the number to show the customer (caller ID), and passes the call to the provider.
- At the provider, the call goes through the gateway and enters the public telephone network.
- The network passes it to the customer's cellular carrier, and the cell phone rings.
- When the customer answers, the confirmation returns the same way, and the voice starts flowing in both directions.
All of this happens within a second or two. From the customer's point of view it is a regular call in every respect — he doesn't know, and doesn't need to know, that it started on the internet.
And for an incoming call — the opposite direction
When a customer dials the organization's number, the public network has to know where the number "belongs." Every phone number is registered with a particular provider, and the network passes the call to it. The provider knows that the number is connected to the cloud PBX, and passes the call in as a SIP call.
From there the PBX decides what to do: play an IVR menu, ring a group of extensions, put the caller in a queue, or send it to a voicemail box. All these decisions happen in the cloud — the public network only brings the call to the door.
This is also why number porting is possible: the registration of the number is moved from one provider to another, and the network starts routing the calls to the new address. The number stays the same number.
What travels along the way: voice, identification and numbers
Crossing between the worlds carries more than voice. Details that accompany the call pass as well:
- The dialed number — where the call is going.
- The caller's number — what will be shown on the screen on the other side.
- The call status — ringing, busy, answered, disconnected.
- Key tones — when pressing keys in another party's IVR menu (see DTMF).
- Fax — which travels over the public network as sounds, and therefore needs special handling in the internet world (see virtual fax).
The voice itself goes through a conversion: on the internet it is compressed with some codec, and on the public network it usually travels in the classic format of 8,000 samples per second. That is why a call between a cloud PBX and a regular phone sounds like a "regular phone" quality, even if inside the organization the voice is better (see HD voice).
The old network and the new network
In the past, the telephone network really was a separate network: copper wires, special exchanges, and a reserved channel for each call. In most of the world, and in Israel too, the core of the network has gradually moved to IP technology. Even the large telephone companies now carry calls inside their networks as data packets.
What remains of the old world is mainly the numbering, the rules, and the connection at the edge: an analog line at home, an analog line at the office, or the cellular network. That is why the difference between an "internet call" and a "regular call" is blurring. The real difference is who manages the call — your cloud PBX, or a telephone company's line.
For the user, this doesn't matter much. What matters to him: that the call rings, that the voice is clear, and that the right number appears on the screen.
Comparison: a call inside the organization vs. a call going out
| Topic | A call between extensions | A call to a regular phone or cell phone |
|---|---|---|
| Does it go through the public network? | No, it stays inside the PBX | Yes, through the provider |
| Sound quality | Can be HD if both devices support it | Usually regular phone quality |
| What is shown on the screen | The extension number or internal name | The organization's external number |
| Depends on the external provider | No | Yes |
| What happens when the provider has a problem | Keeps working | May be affected until the traffic moves to another route |
The table explains something that is sometimes confusing: if internal calls work and outgoing calls don't, the problem is probably not in the phone or in the office network, but in the link that exits to the public network — in the dialing permissions, in the prefix, or at the provider.
Common problems when crossing to the public network
- "The number you dialed is not in service" — sometimes the number really doesn't exist, and sometimes a prefix is missing or the number was entered in the wrong format. It is worth trying to dial from a cell phone and comparing.
- The number doesn't show on the customer's phone, or shows as "private" — caller ID was not set up, or the configured number doesn't belong to the organization, so the provider doesn't pass it on.
- Calls abroad don't go out — on many PBXs, dialing abroad is blocked by default, as protection against toll fraud.
- Calls to special numbers — star numbers, 1-800 and service numbers behave differently and sometimes require a separate permission.
- A fax that doesn't go through — a regular fax over an IP network is sensitive to compression and packet loss, which is why it is connected differently from a phone.
An example from the field
At the "Or HaTorah" yeshiva (a made-up name) there used to be four analog lines from the telephone company. When one line was busy, parents who called heard a busy tone. After the move to a cloud PBX, the main number was ported to a provider that connects it to the cloud.
From the parents' point of view nothing changed: they dial the same number, and it rings. But behind the scenes, the call comes from the public network into the cloud, and from there reaches an IVR menu, the office staff, or a queue — depending on the time and on who is calling. How many calls can come in at once depends on the channels, not on the number of physical lines.
And when the supervisor calls a parent from his extension, the parent sees the yeshiva's main number on the screen, not a random number.
What doesn't depend on the public network
Some things keep working even when there is a problem at the exit to the public network, because they all happen inside the PBX:
- Calls between extensions, even between branches.
- Transferring a call that is already in the PBX.
- Listening to the voicemail box from an extension.
- An internal conference room between extensions.
And some things depend on it directly: every call from or to an external number. That is why a serious PBX is connected to the public network by more than one route, so that a problem with one party doesn't cut the organization off from the world.
A small glossary of terms you will hear
- Gateway — the point where an internet call becomes a call on the telephone network, and the other way around.
- Telephony service provider — the company that holds the numbers and connects them to the public network (see telephony providers).
- Channel — one simultaneous call. A single number can receive several calls in parallel, according to the number of channels.
- Route — the way an outgoing call reaches the network, and sometimes there are several alternative routes.
- Edge line — the last connection to the customer: a line at home, a cell phone, or another organization's PBX.
Before you switch: what to check
Moving from regular lines to a cloud PBX is mainly a change in "who holds the number." To make the move smooth, it is worth preparing in advance:
- A list of all the organization's numbers — the main one, the direct ones, and the fax number.
- With which company each number is registered, and in whose name.
- Which devices are connected to the lines today: phones, fax, alarm, credit card terminal.
- How many simultaneous calls there are at peak hours.
- Which numbers need to keep working even during a power outage.
The third item is especially important. An alarm and a fax don't always move to the cloud the way a phone does, and they require separate planning.
How it works with us at Kesher
Kesher's PBX sits in the cloud, and its connection to the public telephone network is made through the providers, so you don't need a physical phone line at the office. All you need at the office is an internet connection, and the phones connect through it.
Our servers are deployed in several countries — London, Paris, Rome, Athens and Moscow — with backup between them, so a problem in one place doesn't stop the PBX.
An existing number can be ported to us. The porting itself usually takes less than a day, after we have checked that everything is set up and made test calls.
In the control panel, in the call history, you can see every call that came in from the public network or went out to it — including unanswered calls. That makes it easy to check whether a customer's call reached the PBX at all.
And anyone who needs an extension that works even without internet can use a SIM extension — a SIM card that behaves like an extension and runs over the cellular network.
FAQ
Do I need a phone line from the telephone company to work with a cloud PBX?
No. The connection to the public telephone network is made in the cloud, at the provider. At the office you only need internet.
Will my customers notice a difference when I call from the cloud PBX?
Usually not. From their side it is a regular call, with the organization's number on the screen.
Why does an internal call sound better than a call to a cell phone?
An internal call can use a high-quality codec from end to end. A call that goes out to the public network is usually converted to regular phone quality along the way.
Internal calls work, and calls going out don't. What do I check?
The extension's dialing permissions, the number format and prefix, and if there is a wider problem — with the support team, who check the connection to the provider.
What is the difference between PSTN and VoIP?
PSTN is the public telephone network that connects all the numbers. VoIP is the way to carry voice over the internet. A cloud PBX works over VoIP and connects to the PSTN through a provider.
Can I keep my old number?
Yes, in most cases. You port the number, and the public network starts passing its calls to the cloud PBX.
Why does fax cause problems when you move to the internet?
A fax is really sounds traveling over the line, and it is sensitive to compression and packet loss. That is why it is connected in a special way, not like a regular phone.
How many calls can come in to one number at the same time?
It depends on the number of channels that were set up, not on a physical line. On a cloud PBX, a single number can receive several calls in parallel, and the extra ones can wait in a queue.
What happens to incoming calls if the internet at the office goes down?
The calls still reach the cloud PBX, because it doesn't sit at the office. From there they can be forwarded to a cell phone, a SIM extension, or a voicemail box, depending on the configuration.
Will the alarm and the fax move to the cloud too?
They can, but not like a regular phone. They are connected through a dedicated adapter, and it is worth testing them with a test call after the move.