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Telephone Dialing

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 enormously.

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 today.

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 exchange."

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 pull numbers.

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 cities grew.

For further research:

See what were grand visions of telephone services in a 1961 Popular Electronics magazine article.

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.