How an office phone system connects calls
A PBX, short for private branch exchange, is an office's own small telephone exchange. It gives every desk phone a short extension number, connects calls inside the building, shares a few outside lines among everyone and sends each incoming call to the right desk. In the 1990s it was a cabinet of circuit cards, and today it is usually a server on the office network or software in a data centre.
Purple: the call and the parts it usesKey
- Purple: the call and the parts it uses
- Lifted handset: on the phone
- Dashed: the mobile network
Today · 4 desk phones, 201 to 204 · outside line free
Press a button, or click a desk to call that person.Press a button, or tap a desk to call that person.
The same four jobs in both. What changed is the cabinet and its wiring.
Anatomy of a PBX: four calls through a small office and the phone system in its cabinet, as it works today and as it worked in the 1990s. 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
You dial a colleague's extension
You lift the handset on your desk and dial 203, Eva's extension. Your phone sends the three digits down its cable to the phone system in the cabinet by the wall. Today that cable is a network cable, and the network switch in the cabinet passes the digits on. In the 1990s every desk had its own pair of copper wires to a line card in the cabinet.
How does the phone system know where Eva's phone is?
Today each phone tells it. When a phone starts up, it sends the phone system a short message, called a registration, with its extension and its address on the network. It sends it again from time to time. So Eva can plug her phone into any network socket in the building.
In the 1990s the wiring told it. Eva's pair of wires ended on one port of one card, and moving her desk meant rewiring that pair. That wiring, one pair to every person, was a large part of what a phone system cost to install, says the UK's National Cyber Security Centre.
Sources: IETF RFC 3261: SIP, the protocol of internet calls (2002); National Cyber Security Centre: private branch exchange best practice (2024)
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Step 2 of 5
The phone system rings Eva and connects you
The phone system looks up 203, finds Eva's phone and rings it. When she picks up, it connects her phone to yours. The call never leaves the building. It uses none of the outside lines, and no phone company bills it. Look inside the cabinet below.
Inside the cabinet The phone system
- 1Patch panel
- 2Network switch
- 3Phone system
- 4Router
A program on one server does the switching that a shelf of circuit cards did in the 1990s.
- 1Line cards
- 2Control cards
- 3Trunk card
- 4Shelf
30 calls at once could share the trunk card's one ISDN primary rate line.
What does the connecting inside the cabinet?
In the 1990s, circuit cards did. Each call got its own time slot, a moment that came round thousands of times a second, when the cabinet passed a sample of one voice across to the other phone. Bell Labs already switched office calls this way in its electronic phone system of 1963.
Today a program on a server does it. It sets up the call, and the two phones can then send their voices to each other over the office network while the phone system keeps track of the call.
Sources: Bell System Technical Journal: time division switch units for No. 101 ESS (1969); IETF RFC 3261: SIP, the protocol of internet calls (2002)
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Step 3 of 5
You press 9 for an outside line
To phone a customer, in many offices you press 9 first and then their number. The 9 tells the phone system the call is for outside, so it takes a free outside line and passes the number on to the public phone network. Today that line is a SIP trunk, which runs over the office's internet connection through the router in the cabinet. In the 1990s it was a cable from the telephone exchange, wired through the frame on the wall to the trunk card in the cabinet.
Why not give every desk its own outside line?
Because people seldom all phone out at once. Bell Labs' electronic phone system for offices of 1963 served up to 200 extensions with at most 40 outside lines. The chart below shows how few lines an office of 40 people needs.
The outside lines are also what criminals want. Once they break into a badly protected phone system, they dial expensive international or premium-rate numbers hundreds or thousands of times, and the office pays the bill, the UK's National Cyber Security Centre warns.
Sources: Bell System Technical Journal: time division switch units for No. 101 ESS (1969); National Cyber Security Centre: private branch exchange best practice (2024)
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Step 4 of 5
A customer calls your direct number
Now a customer dials your own number at work. The call arrives on one of the outside lines, and the public network sends the last digits of the number with it. The phone system matches them to extension 201 and rings your phone. No receptionist answers first. This is direct dialling-in, or DDI.
How did a call reach your desk before direct dialling-in?
Through an operator. Every call to the company's one number rang at a switchboard, and the operator asked who you wanted and plugged a cord into that extension's socket. On the switchboard Buckingham Palace put in in 1930, a lamp lit up whenever someone in the Palace lifted a handset.
The International Telecommunication Union set out direct dialling-in for digital lines in 1988. In its words, the caller reaches a person on a private system without attendant intervention. There is no operator in between.
Sources: London Museum: Buckingham Palace private branch exchange (1930); ITU-T I.251.1: direct-dialling-in (1988, revised 1992)
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Step 5 of 5
You pass the call to Ben
The customer needs Ben. You press Transfer and dial 202. The phone system puts the customer on hold and rings Ben. You can speak to Ben first or hang up at once. Either way the phone system connects the customer to Ben's phone, and your line is free again.
Does the call still run through your phone?
No. Your phone only tells the phone system where the call should go next. From then on the customer's call runs from the outside line, through the phone system, to Ben's phone.
On an internet phone system the message that does this is called REFER. The standard names both ways of doing it. In a blind transfer you hang up before Ben answers. In a consultative transfer you speak to him first.
Inside the cabinet: the 1990s and today
The phone system does the same four jobs in both. What changed is the wiring to the desks, the line to the outside and what does the switching.
The same jobs, taken out of the cabinet and laid out in the order a call meets them. Point at a part, or tap it, to see what it does.
Cabinet, 1990s
Server, today
The phone system
- 1Line cards
- 2Control cards
- 3Trunk card
- 4Shelf
30 calls at once could share the trunk card's one ISDN primary rate line.
The phone system
- 1Patch panel
- 2Network switch
- 3Phone system
- 4Router
A program on one server does the switching that a shelf of circuit cards did in the 1990s.
How a desk phone connectsNational Cyber Security Centre
Copper pair
Its own pair of wires to a card in the cabinet
Network cable
The same network the computers use
What connects the callBell System Technical Journal; IETF RFC 3261
Time slots
Circuit cards pass samples of each voice across
Software
A program on a server sets the call up
The line to the outsideITU-T I.431; National Cyber Security Centre
30 calls
At once, on one ISDN primary rate line of 2,048 kbit/s
SIP trunk
Over the office's internet connection
Moving a deskNational Cyber Security Centre; IETF RFC 3261
Rewire it
Run the desk's pair of wires to its new place
Plug it in
The phone registers from any network socket
How many outside lines an office needs
Take an office of 40 people. In its busiest hour, each of them spends about six minutes on calls to the outside. The chart shows how often someone presses 9 and finds every outside line in use.
The chance that every outside line is busy, by the number of lines
Worked out with Erlang's formula for lost calls, from 1917. It counts a caller who finds every line busy as one who gives up and tries again later.
Elektroteknikeren
How it came to be
- 1930
A switchboard for the Palace
Buckingham Palace puts in a new private branch exchange, worked by operators who plug in a cord for every call. It weighs 715 kg and stays in use until the late 1960s.
London Museum: Buckingham Palace private branch exchange (1930)
- 1963
Electronics take over
Bell Labs completes its No. 101 electronic switching system for office phone systems. A program in the telephone company's building runs it, and a unit on the customer's site connects calls by time slots, for up to 200 extensions.
Bell System Technical Journal: time division switch units for No. 101 ESS (1969)
- 1988
Your own number at work
The International Telecommunication Union sets out direct dialling-in for digital lines. A call goes straight to a person's extension, and no operator answers it first.
- 2002
Calls on the office network
SIP, the standard internet calls run on, is published. A phone system can now be a program on a server, and desk phones plug into the same network as the computers.
- 2003
Power down the network cable
The IEEE publishes 802.3af, which lets a network cable carry power as well as data. A desk phone on the office network no longer needs a plug of its own.
- 2018
911 without the 9
The United States passes Kari's Law. From 16 February 2020, a new office phone system must let anyone dial 911 directly, without the 9 for an outside line, and must alert someone in the building, such as the front desk.
Where Voxbi fits
Voxbi is a cloud phone system hosted in the European Union, and it does these jobs from a data centre: extensions, outside lines, direct numbers and transfers.
See it in Voxbi