How a phone system works from a data centre
A cloud PBX, also called a hosted PBX, is an office phone system that runs as software on servers in a data centre instead of in a cabinet in the office. The desk phones stay on the desks and reach it over the office's internet line. Every call is set up in the data centre, even a call to the next desk, and the outside lines start there too.
Purple: the call and the parts it usesKey
- Purple: the call and the parts it uses
- Lifted handset: on the phone
- Dashed: the internet and the mobile network, far longer than drawn
Phone system in the data centre · 4 desk phones, 201 to 204
Press a button, or click a desk to call that person.Press a button, or tap a desk to call that person.
Same phones, same jobs. Only the phone system moved, into the data centre where the trunk already was.
Anatomy of a cloud PBX: the office from the PBX page, with its phone system moved into a data centre, and four calls between them. The switch puts the phone system back in the office cabinet. The drawing is not to scale.
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Follow one call
Press Play to watch the call, or scroll down and the drawing follows you one step at a time. Try it, under the drawing, makes your own call.Press Play to watch the call, or Next to go one step at a time. Try it, under the drawing, makes your own call.
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Step 1 of 5
Your desk phone signs in to a phone system far away
This is the office from the PBX page. The desks, the desk phones, the cables and the cabinet are all still here, but the server that ran the phone system has gone from the cabinet. The phone system now runs as a program on a provider's servers in a data centre. When your phone starts up, it sends a short message through the network switch, the router and the office's internet line to the data centre. The message says that extension 201 is here, and it is called a registration. Look inside the data centre below.
Look inside The phone system
- 1Router
- 2Phone system
- 3Trunk
- 4Storage
150 milliseconds one way, mouth to ear, is the delay below which most calls are not held back. Many routes of up to 5,000 km within one region stay under it.
- 1Patch panel
- 2Network switch
- 3Phone system
- 4Router
1 server in the office cabinet runs the phone system, and the office looks after it.
How can a server far away ring a phone behind the office router?
An office router lets traffic in from the internet only when it answers something sent from inside. If nothing passes for a while, the router forgets the way back, and a silent phone would drop out of reach.
So the phone keeps the way open. Every so often it sends a small message, and the phone system answers. The IETF, which writes the internet's standards, set this out in 2009. Another of its standards tells routers to keep such a way open for at least two minutes with no traffic.
Sources: IETF RFC 5626: SIP connections that start from the phone (2009); IETF RFC 4787: network address translation and UDP (2007)
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Step 2 of 5
You call Eva, two desks away
You lift the handset and dial 203. The digits leave the building, out through the router and the internet line to the phone system in the data centre. It looks up 203, finds the phone that signed in as Eva's and rings it, back over the same internet line. Eva sits two desks from you, and the call was set up somewhere neither of you can see. Switch the drawing to the office cabinet for the old way. There the call never leaves the building.
Does your voice go to the data centre too?
It depends on the phone system. It always sets the call up. Once it has, the two phones can send their voices straight to each other across the office network.
A phone system can also carry the voices through the data centre. The IETF's description of such servers names both jobs. One only sets calls up, and the other, a media relay, passes the voice on as well.
Sources: IETF RFC 3261: SIP, the protocol of internet calls (2002); IETF RFC 7092: the roles of servers in the middle of a SIP call (2013)
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Step 3 of 5
You phone a customer
Now you dial a customer's mobile. The phone system in the data centre takes one of its outside lines and passes the number to the public phone network, which rings the customer. Those outside lines no longer reach your office. They start in the data centre, as a trunk to the telephone network, and your office's internet line carries the call only as far as the data centre.
What happens when someone dials an emergency number?
The call goes out like any other. But a phone system in a data centre cannot see your desk. It knows your phone by its registration, and a registration says nothing about the street, the floor or the room. Someone has to tell the phone system where each phone is.
In the United States the rules now say so. Since January 2021 an office phone system has to send a dispatchable location with every 911 call from a desk phone. That is the street address, plus the floor or room where it is needed.
Sources: Federal Register: FCC rules on 911 calls from office phone systems (2019)
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Step 4 of 5
A customer calls your number
A customer dials your direct number. The call reaches the data centre through its trunk, because that is where your number lives now. The phone system finds the phone that signed in as 201 and rings it over the office's internet line. Your desk phone rings as it always did, and nothing on it shows that the call came the long way round.
What if the office's internet line goes down?
Then your desk phones lose the phone system, because they reach it over that line. The phone system keeps running in the data centre, so a customer who calls still gets an answer. It follows its rules for a phone that does not answer, such as a voicemail box or a mobile number.
With the phone system in the cabinet it was the other way round. Calls between the desks still worked, and no call from outside could get in.
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Step 5 of 5
Your voice crosses the internet line in packets
You answer. Your phone cuts your voice into slices of 20 milliseconds and sends each one as a small packet, 50 a second, out through the router and over the internet line to the data centre. The customer's voice comes back the same way, 50 packets a second. A call needs little of the line's speed. What it needs is a short delay that stays short.
How far away can the data centre be?
Far enough to cross a country, and more. What you notice on a call is delay, the time your voice takes to reach the other ear. The International Telecommunication Union, the United Nations agency for telecoms, sets the mark at 150 milliseconds one way, mouth to ear. Below it, most calls are not held back by the delay.
It also says that on many routes of 5,000 km or less within one region, such as Europe or North America, internet calls are likely to stay under that mark. A data centre in the same country as your office is well within that distance.
Look inside: the office cabinet and the data centre
The desk phones, the cables and the jobs are the same in both. What moved is the phone system, and the outside lines went with it.
The parts a call meets, laid out in order. In the office cabinet the phone system is a server of its own; in the data centre it is a program on servers in a rack, run by the provider. Point at a part, or tap it, to see what it does.
Office cabinet
Data centre
The phone system
- 1Patch panel
- 2Network switch
- 3Phone system
- 4Router
1 server in the office cabinet runs the phone system, and the office looks after it.
The phone system
- 1Router
- 2Phone system
- 3Trunk
- 4Storage
150 milliseconds one way, mouth to ear, is the delay below which most calls are not held back. Many routes of up to 5,000 km within one region stay under it.
Where the phone system runsNational Cyber Security Centre
Office
A server in the cabinet, looked after by the office
Data centre
A provider's servers, reached over the internet
A call to the next deskIETF RFC 3261
Stays inside
The server in the cabinet sets it up
Out and back
Set up in the data centre, over the internet line
Where the outside lines startNational Cyber Security Centre
Office router
A SIP trunk over the office's internet line
Data centre
A trunk from there to the phone network
If the office's internet line goes downNational Cyber Security Centre
Inside calls work
No call from outside gets in
Desk phones stop
The phone system still answers callers
Security updatesNational Cyber Security Centre
The office
Its own staff or supplier patch the server
The provider
Check that it updates the system by itself
How many calls an office internet line carries
A call sends 50 packets a second in each direction. Each packet holds 160 bytes of voice and 40 bytes of addresses and numbering, so a call needs 80,000 bits a second each way. The chart divides the speed of a line's slower direction, usually the upload, by that.
Calls at once, by the speed of the line's slower direction
Worked out for G.711, the phone network's standard way of turning a voice into numbers, with nothing else on the line. The line's own framing takes a little more, so leave room to spare.
How it came to be
- 1963
The brain leaves the building
Bell Labs completes its No. 101 electronic switching system for office phone systems. The program that runs it sits in the telephone company's building, and only the unit that connects the calls stays with the customer.
Bell System Technical Journal: time division switch units for No. 101 ESS (1969)
- 1996
Calls on computer networks
The International Telecommunication Union approves H.323, a standard for calls over local computer networks that do not guarantee their quality.
- 2002
Phones that sign in
SIP, the standard internet calls run on, is published. A phone tells a server where it is by registering, so the server can sit anywhere the phone can reach.
- 2003
Voice in 20-millisecond slices
The IETF sets out how to carry voice in packets. Unless the two ends agree otherwise, each packet holds 20 milliseconds of sound.
- 2009
Through the office router
The IETF publishes a way for phones behind an office router to keep their connection to a server far away open, so the server can ring them at any time.
IETF RFC 5626: SIP connections that start from the phone (2009)
- 2019
Where is the caller?
The US Federal Communications Commission rules that office phone systems must send a dispatchable location with 911 calls. That is the street address, plus the floor or room where it is needed.
Federal Register: FCC rules on 911 calls from office phone systems (2019)
- 2024
Advice for cloud phone systems
The UK's National Cyber Security Centre publishes advice for office phone systems. For a cloud phone system, it says to check that the provider updates it automatically and encrypts the calls.
National Cyber Security Centre: private branch exchange best practice (2024)
Who is allowed to connect your calls
A phone system that gives you phone numbers and calls to and from the public phone network is a telecom service in law, not only software in a data centre. In the European Union, the European Electronic Communications Code sets the rules, and each country's telecom regulator enforces them. An app that calls only other users by name, with no phone numbers, has fewer duties and does not have to notify a regulator.
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It tells the regulator before it starts
The Code calls a service whose phone numbers reach the public network a number-based interpersonal communications service. Its provider needs no licence to wait for, but a country may ask it to notify the regulator before it starts. Luxembourg and Belgium both do, and Luxembourg's regulator keeps a public register of every provider that has.
EUR-Lex; Institut Luxembourgeois de Régulation; Belgian Institute for Postal Services and Telecommunications
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It connects emergency calls, with your location
Every user must be able to reach the emergency services on 112, free of charge. The provider passes the caller's location to the emergency centre as soon as the call is set up. Step 3 above explains why a phone system in a data centre has to be told where each desk phone is.
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Your numbers go with you
Anyone with a number from a national numbering plan has the right to keep it when they move to another provider. Once the move is agreed, the new provider must have the number working within one working day of the agreed date.
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It keeps the service secure, and open to the law
The provider must manage the risks to the security of its network and services, with encryption where it is appropriate. A country may also require it to make legal interception possible for the national authorities that have the power to order it.
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Other countries have their own rules
Outside the European Union, other laws apply. In the United Kingdom, Ofcom, the communications regulator, sets general conditions that can bind every provider of a communications service. In the United States, the Federal Communications Commission sets rules such as the 911 duties in step 3.
Where Voxbi fits
Voxbi is a cloud phone system hosted in the European Union, on our own infrastructure in Luxembourg. Your desk phones and the Voxbi apps sign in to it over the internet. It runs on Mixvoip, a telecom operator overseen by the Institut Luxembourgeois de Régulation, the Belgian Institute for Postal Services and Telecommunications, France's Autorité de Régulation des Communications Électroniques et des Postes and Germany's Bundesnetzagentur, among other regulators.
See it in Voxbi