| Field | Telecommunications |
| Went Obsolete | 1996 |
| Made Obsolete By | Local number portability (LNP-the ability to assign a phone number to a line on a different exchange or carrier) |
| Knowledge Assumed | The ability to notice patterns in exchanges and remember them |
| When useful | Phone numbers that have not been ported to a line on a different exchange or to another carrier |
The exchange (or prefix) of a telephone number used to be specific to an exchange, or central office (which connects you to other exchanges, and the rest of the global telephone network). That was why if you moved, even within a city, you might have been assigned a new phone number.
This started to change in 1996 when the FCC began to require LNP.
If you have had your phone number at the same place since before 1996, your exchange will, of course, still reflect the part of town in which you live.
Going back even further, up until the early 1970s, it was common for people to give the telephone exchange number in place of the first two digits of their seven-digit phone number. The exchange name often was based on the street where the exchange was located. For example, a person whose phone number was 737-4031 would typically give out their phone number by saying “Pershing 7-4031” (on the phone dial, the letter 'P' was on the '7' and the 'E' was on the '3'.) Some people would shorten this to PE7?-4031. This all went away long before 10-digit dialing, but that's a whole other topic of discussion.
People eventually knew exchange names belonged to certain parts of the city and made associations and assumptions based on your telephone number. The exchange names not only identified the area of a large city but often the income level and ethnicity of the person. If you asked a girl for her telephone number, you immediately had a hint about whether the girl's family or she had money, lived in the “good” or “bad” neighborhoods, and given you were unclear about her ethnicity, you had a means to confirm a guess. People often moved from one area of a city to another so that they or their children could have “high status” exchange names.
“Did you live in downtown San Francisco? Or were you out by Golden Gate Park? Or near the Marina? Your telephone number gave a clue. All number dialing wiped out all these names and the memories that went them, much angst ensued, and countless editorials mourned their loss. Witness this lament from New York City: 'You could learn about a fella by knowing his exchange. A MOnument fella was up near 100th Street and West End Avenue. You could picture him coming downtown on the IRT, strolling first to 96th and Broadway for the newspapers, passing the Riviera and Riverside movie theaters (both gone). The ATwater girl was an East Side girl, a taxi-hailing girl, on her way to her job at Benton and Bowles. A CIrcle fella was a midtown fella, entering his CIrcle-7 Carnegie-area office with a sandwich from the Stage Deli. And what about a SPring-7 girl, twirling the ends of her long brown hair as she lay on her bed talking to you on the phone? A Greenwich Village girl. A 777 girl is nothing. She is invisible. She is without irony, seldom listens to music.' (cited in http://www.answering-services-phone-messaging.com/history_telephone_numbers.html)
Originally, the use of exchange names was thought to aid in remembering and dialing phone numbers, later human factor studies determined that this was not the case. However, from the 1930's until the mid 1980's exchange names were a part of the social landscape. The onset of the civil rights movement in the mid 1960's may have contributed to the decision to use all numeric telephone numbers as much as the need for additional exchange combinations. In that time, some employers made hiring decisions on the basis of the applicant's exchange name without the need for an interview. The elimination of exchange names may have helped end some forms of discrimination, but unfortunately there are others such as high school or college attended.
Some of the historical exchange names can be found at: http://ourwebhome.com/TENP/Recommended.html
The younger generations may be able to observe the lingering influence of these social cues in the memories of their grandparents or great-grandparents. Most people over 50 years of age have memories about some of the various exchange names, particularly their own and those of long-lost acquaintances.