How a call finds the right person
Call routing is the set of rules that decides which phone rings when you call a company. First a recorded voice, the IVR (interactive voice response), asks you to press 1 for sales or 2 for support. Then the phone system tries that team's phones one at a time, and puts you in a queue if everyone is busy. The company sets every one of these rules. Today the phone system usually runs in a data centre, not on the office wall, and the rules work the same way.
Purple: your callKey
- Purple: your call
- Dark screen: on another call
- Mobile in line: a caller on hold
- Dashed: mobile network or internet
Office open · 2 of 8 people on other calls
Press a button, or click a desk to put that person on another call.Press a button, or tap a desk to put that person on another call.
Same rules in both. Only the phone system moved.
Anatomy of call routing: one call through a small office, as it works today and as it worked in the 1990s.
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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
The voice menu (IVR) asks what you need
Your call reaches the company's phone system before it reaches any person. Today that system runs in a data centre and reaches the office over the internet, through the router on the Reception wall. A recorded voice reads the options, and you press 1 on your phone for Sales. That one key press decides which team's phones ring.
How does the menu know you pressed 1?
Each key sends two tones at once, one low and one high, and every key has its own pair. A keypad uses eight tones in all, between 697 and 1633 Hz. The phone system listens for the pair and knows the digit.
On an internet call the press often travels as a short labelled message instead, because compressed sound can blur the tones.
A menu does not remove the need for people. At one bank's call centre, in a full year of its calls in 1999, 35% of callers still asked for a person after the menu. The other 65% got what they needed from the menu itself.
Sources: ITU-T Q.23: push-button telephone sets; IETF RFC 4733: key presses in internet calls (2006); Journal of the American Statistical Association: a telephone call centre study (2005)
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Step 2 of 5
Sales tries one phone at a time
Sales is a hunt group: four phones tried in an order the office chose, here Anna, Ben, Eva, then Finn. Anna is already on a call: her handset is at her ear and her screen is dark. So the system skips her phone without ringing it and tries Ben.
Why is it called hunting?
Old telephone exchanges did this job with a switch that stepped from contact to contact, hunting for a line that wasn't busy. The name stuck.
Trying phones in the same order every time means Anna always gets the most calls. A phone system can also start after whoever took the last call, or ring every free phone at once.
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Step 3 of 5
Each phone gets 15 seconds
Ben's phone rings, and the clock on the wall times it. The office set the limit at 15 seconds. If Ben picks up, you're through. If he doesn't, your call moves on to the next free phone.
Why 15 seconds?
It's a trade-off. Too short, and people miss calls they were reaching for. Too long, and you wait at an empty desk while someone two desks along is free.
In much of Europe the ringing you hear is one second on and four off, so 15 seconds is three rings.
Sources: ITU-T E.180: telephone tones
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Step 4 of 5
Everyone's busy, so you wait in line
Now take a busier moment: Anna, Ben, Eva and Finn are all on calls. Rather than drop you, the system puts you on hold, drawn here as a mobile waiting in line. Callers wait in the order they called. Many systems play music and tell you your place in line.
How long will people wait?
It depends on how many calls come in and how many people answer. Say calls last three minutes and 80 arrive an hour. With five people answering, 55% of callers wait. A sixth person cuts that to 29%, and the average wait from about 100 seconds to 26.
The maths comes from Agner Krarup Erlang, who worked it out in 1917 at the Copenhagen Telephone Company. Call centres still plan their staff with it.
The waiting message matters too. In a year of calls to a bank's call centre, hang-ups jumped in the first seconds, right after "please wait" first played, and again when it played a second time, about a minute in.
Sources: Erlang C formula (1917), computed by Voxbi; Journal of the American Statistical Association: a telephone call centre study (2005)
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Step 5 of 5
First in line gets the next free phone
Finn finishes his call. You're first in line, so your call goes straight to his phone, it rings, and he answers. Everyone behind you moves up one place. Today Finn takes that call on his mobile at home: to the phone system, it's one more Sales phone.
Do you have to stay on the line?
Not always. Many queues offer a callback: you hang up, keep your place, and the system rings you when a phone is free.
Large call centres run all of this in an automatic call distributor, which hands each queued call to whichever agent is free.
How long you wait
Take Sales at a busy hour: 80 calls an hour, each about three minutes long. Whether you wait, and for how long, depends on how many people answer.
Your chance of waiting, by the number of people answering
Worked out with Erlang's formula from 1917, which call centres still plan their staff with. The average counts every caller, including those who get straight through.
Elektroteknikeren
How people get in touch
78 in 100
In 2024, when UK mobile customers contacted their provider, 78 times in 100 they phoned. Broadband and landline customers phoned 86 times in 100. Webchat came a distant second, at 14.
Who gives up
1 in 15
In 2024, about 1 in 15 calls to UK phone and broadband providers ended with the caller hanging up before reaching a person. The average wait was about two minutes.
How it came to be
- 1891
Calls without an operator
Almon Strowger, an undertaker in Kansas City, patents an automatic telephone exchange. For the first time, a machine connects calls with no operator in the loop.
- 1917
The maths of waiting
Agner Krarup Erlang publishes the maths of how often a call finds every line busy and how long a caller waits. Queues are still planned with it.
Elektroteknikeren: Erlang on telephone traffic (1917)
- 1963
Keys a machine can hear
The Bell System starts selling push-button Touch-Tone phones in two Pennsylvania towns. Each key sends tones a machine can hear, which makes voice menus possible.
- 1964
A computer answers the phone
IBM's 7770 Audio Response Unit answers callers in spoken words: a caller keys in a question and a computer reads out the answer. Menus that route calls to people come later.
- 1992
Say it, don't press it
AT&T announces it will handle collect calls by speech recognition: callers say "collect" to a machine instead of an operator. It expects to cut 3,000 to 6,000 of its 18,000 operators.
Christian Science Monitor: AT&T and speech recognition (1992)
- 2002
The phone system moves online
SIP, the standard internet calls run on, is published. Phone systems can now run as software in a data centre instead of a box on the office wall.
- 2025
A person in the menu, by law
Spain passes a law for large companies and essential services: the main voice menu must offer a person from the first moment, and 95% of calls must be answered in under three minutes on average. Companies have a year to comply.
Boletín Oficial del Estado: Spain's customer service law (2025)
Where Voxbi fits
Voxbi, a cloud phone system, lets you build voice menus, hunt groups and queues in a visual editor and change them yourself.
See it in Voxbi