Mar 31, 2015

Engineering Explains It All: The Numbers on Your Phone

What can engineering teach you? Interlace it with history and it can explain the world as you know it (or knew it).

I was having lunch with a friend and somehow phones came up. Suddenly, she said, "I never understood how that switchboard thing worked."
Image by Dan Shirley, via

So I explained that in the old days, phone were for the rich, with a direct line from one place to another, like the drawing room and the servants' quarters. If you wanted to connect two other places, you bought another phone line, which would connect, say the master bedroom to the servants' quarters. So some places, like the servants' quarters, would have multiple phone lines. This didn't seem weird as that's how the old bell system worked, when you would ring for the butler.

If one rich person wanted to connect to another rich person, then a line was set up from one house to another. Eventually, when more phones became prevalent, and one person wanted to anyone who happened to have a phone, the switchboard idea came into play. Basically, all the phone wires would come into the switchboard. Simply put, the switchboard provides a human a way of connecting two phones together so they can communicate.

Generally, speaking, a person would turn a hand crank on the phone to "ring" the operator at the switchboard. She (usually a she) would insert a plug into your line and connect so she could talk with you. Then she would ask you who you wanted to talk with. If you wanted to call up the grocer, then she would plug go to the grocer line, ring that phone line and make sure there was someone there. Then the operator would plug a wire into your line and the grocer's and thereby connect you.

A small town switchboard could have a number of lines coming in. If you wanted to call someone in another town, the operator would connect to that outside switchboard's operator. That operator would connect with the person you were calling, and then wires would be plugged into connect the phones up.

If you think of a city as a series of small towns, you can see that giving each phone a number, rather than a name, may make things easier for an operator in your area of the city. But if you wanted to call another area, you would also need to indicate the area of town you were trying to call in addition to the phone's number. Hence, the movie titled "Call Northside 777" referred to a phone number. Eventually, there were too many areas, and automated switching was developed where people could get automatically connected by typing in the number for the area, followed by the number for the phone. But how would people know which three numbers to enter?

Enter the idea of letters on the phone. With our example, Northside, you would find the first three letters on the phone (NOR) and press the corresponding number: 667. This was known as the exchange number, and this protocol explains why the old land lines in the same area of town often have the same first three numbers. We started to lose this idea as more phones came up in an area, and numbers that didn't correspond to the area name were added.

Other dialing protocols can be explained by the automatic switching, too. For example:

  • Dialing 1 for a long distance call
  • Area codes (in the old days) having a 0 or 1 as the middle digit

For those of us who remember landline phone numbers, this really sheds light on our world, and thereby gives us an instinct for reverse engineering phone switching technology. Pity the poor millennial who not only doesn't have this history to intuit how automated systems work, but also doesn't know why phones "ring"...

See also:

  • North American Numbering Plan, Wikipedia for explanations on the rules around phone numbers in the old days
  • Switchboards, Old and New, At&T Archives for pictures of the old switchboard systems and some trivia around the switching system (e.g. boys were deemed poor operators because of the pranks and cussing they did with the callers)