What are all the digits in a telephone number and where did they come from? Why is
it called "dialing" the phone, anyway? Why is there a "dial tone" in land lines?
The history of automated telephone call switching is rich with engineering optimizations
and a transition from electro-mechanical to electronic systems just in time for an
explosion in demand. It is a success story of a neat system that evolved over time
to serve the industry extremely well even as the number of end points expanded enormously,
as any well-planned engineering endeavor should.
<a work in progress>
Toll telephone call routing in the United States and Canada is steered by a three-digit
Area Code that tells the switching office the geographic area in which to find the
ultimate destination. Originally, any "long distance" call was initiated by dialing
a 1 (later globally known as the "country code"), followed by the area code, then
the desired local exchange, and the final four digits identifying the destination
instrument - "subscriber" in telephone parlance.
Direct Distance Dialing (DDD) was an early name of the technique that allowed users
to call a telephone that was in a different service area and would incur "long distance"
charges. DDD allows the user to initiate the call through dialing without requiring
a human "operator" to intervene to either set up or take down the connection. Two
important related features were critical to this automated process. One was a uniform
numbering system that inherently identified both the region to be called and the
desired telephone set. The second was equipment that can quickly decode these numbers,
route the call, connect the circuit end-to-end, and monitor the call so it could
disconnect the call when the conversation was over.
The Bell Telephone System (which later became American Telephone and Telegraph -
AT&T) adapted a scheme that could give every telephone in the country a unique identifying
number that people could remember and dial easily enough (though the ability to recall
seven digits, let alone ten, was questioned early on.) Also, equipment of the day
had to be able to separate the shorter local phone numbers from longer toll (charged)
numbers, and properly route the calls. Establishing the numbering system gave the
framework around which people and equipment could work. That system evolved over
time and has served the industry quite well even as the number of end points expanded
Area codes were just one part of the number coding scheme, but the blending of these
seemingly random numbers with the underlying technology reveals what excellence in
engineering is really all about. Indeed, history may be deeply embedded in everyday
tasks we perform – like dialing the phone.
The discussion here is based on glimpses of telephony in the United States in what
might be considered its wired heyday – the 1970's, when AT&T was a long-established
highly-functional government-sanctioned monopolistic utility. Early days of "cord
boards" and operators did not really have a significant impact on today's telephone
numbers or dialing, nor did the breakup of "Ma Bell", so such earlier and later events
are not included here. This discussion will center on how we got the numbers we have
Dial Pull – Efficiency in Numbers
One thing about a proprietary system is that a single strategy that best benefits
the whole system can be effectively implemented. The Bell System, affectionately
known as "Ma Bell", sometimes merely referred to as "TPC" - the phone company - was
so engrained as a monopoly that Bell once had an advertising moniker it shortened
to WMBTOPCITBWTNTALI - We May Be The Only Phone Company In Town But We Try Not To
Act Like It. Some may question why a guaranteed monopolistic business needs to advertise
at all but this discussion will try not to stray into politics.
The researchers and engineers at Bell Labs and the manufacturing arm Western Electric1
worked together for the Bell System designing equipment that would operate with a
design life of 40 years – something unheard of today. The 500 series telephone set
placed in the home in 1950 should still operate in 1990. Most did. The equipment
in the central switching offices (CO) also was expected to last that long. Since
the Bell System was virtually assured of being in business eternally, it made sense
to install the equipment once and almost never have to touch it again. From the springs
in the dials and the color and hardiness of the cases to the investment in cabling
and repeaters to the automated switching equipment clattering away all through the
night, the life span was to be 40 years.
A critical piece of equipment used to connect a phone call is the switch. A "rotary
stepping switch" was the fundamental mechanism for connecting an incoming call to
its destination in early automated telephony. A stepping switch is purely electromechanical,
using solenoids, relays and cogs to perform a two-step process to select which of
maybe 100 outgoing lines to connect to the incoming line.
They were operated by the regular pulses of direct current (DC) which would step
up a level or step around one position for each pulse. The telephone number dialed
represented the number of pulses generated so that dialing a 4 followed by a 7 would
step up 4 levels and rotate around 7 positions. Rotary steppers can be thought of
as a stack of 10 rotary switches, built around a vertical rod around which one or
two wipers rotate in a half circle to connect the circuit to wires that would lead
to the desired phone.
Connecting a series of these mechanical switches configured as line-finders/hunters,
selectors, and connectors would eventually route the call to the ultimate destination,
possibly across the country.
Buildings full of switching equipment and attached to one end of tens of thousands
of wires are the central offices (CO), sometimes referred to as a telephone exchange.
The exchanges were given names, like Alpine, Capitol, Glendale, Greenwood, and Homestead,
initially so that switchboard operators could identify them. These names later transitioned
into numbers associated with the first two letters of the exchange. Today the word
exchange often includes the third digit, and may be all numbers, such as "the 345
One can see that the CApitol (22) exchange would be less stressful on the switching
equipment than the GReenwood (47) exchange. Many exchanges like Capitol and Central
are located downtown, because that is likely where the city originated and early
telephone systems were set up there, taking advantage of the short dial pull. Perhaps
more significant is the greater likelihood of a high concentration of businesses
downtown. More people are going to call the department store, restaurant, or bank
than will call Aunt Mabel so it is best for businesses to be assigned the short dial
A key factor underlying the switching scheme was that each mechanical motion consumes
time and electricity while wearing down the equipment. To minimize maintenance, one
would want to minimize these dial pulses. Observe that telephone numbers consisting
of the lowest numbers (1, 2, 3) generate fewer pulses and would not wear out the
equipment as fast as higher numbers (8, 9, and 0 – zero on the original telephone
dial follows the 9) that cause up to ten times the wear-and-tear on the equipment.
The "dial pull", or number of pulses created by dialing each digit, directly affects
the wear on the electro-mechanical switching equipment. To reduce such wear, telephone
numbers should be assigned starting with the lowest digits. Notice that this does
not mean the lowest numbers. While the number 4680 is lower than the number 5432,
4680 would cause 28 (4+6+8+10) solenoid movements while the slightly larger 5432
would cause exactly half as many actions. Thus, telephone numbers containing lots
of 1's and 2's are desirable.
There is another way to limit the dial pulses required to connect to another phone.
Fewer digits in each telephone number require fewer switches. Early telephone numbers
were quite short, maybe only four digits long. But the number of digits directly
determines the number of users that can be serviced and four digits allows connections
to only 10,000 end points. As telephone subscribers grew, more digits were required
to identify each line and the equipment to support them had to grow as well. It is
only in modern society that seven digits (ten million end points) are insufficient
to service a major metropolitan area, causing additional area codes to be added as
For further research:
See what were grand visions of telephone services in a 1961 Popular Electronics magazine
In the Glossary, see terms such as: PBX
1 Interesting note: Western Electric was an entity long before the Bell System organized.
In fact Western Electric produced the equipment used by the well-known US telegraph
company Western Union (…so that's where the "Western" comes from…), Western Union
having been formed as a conglomeration of numerous once-independent telegraph operators.