In the beginning, phone companies used names and not numbers to connect callers. As more and more people started using telephones that became too confusing to build a telephone infrastructure, as many people in a town might share the same name. Starting in 1879 the switch to assigning each customer a number began, typically using a four-digit code. All calls were still connected by a human operator.
AT&T’s operating companies started installing dial telephones in the mid to late 1920s. Customers could now dial numbers rather than having an operator connect calls. Rather than use all digits to indicate a telephone number, AT&T began a hybrid system of letters and numbers. Instead of a number like 675-9076, the Bell System referred to it by a name like LA0-9876 or KC2-3498. The two letters and a number indicated a customer’s switching office or exchange, the last four digits the actual customer’s number.
In January of 1958, Wichita Falls, Texas was the first American city to put in ANC or true number calling, which has seven numerical digits without letters or names. After 15 years, ANC replaced the system of letters and numbers begun forty years before at the advent of the automatic dial.
Telephone numbers are divided into two parts. Four digit codes allowed 9,999 possible telephone numbers. That is enough for small towns but not for big cities. Each 9,999 telephone has a three-digit code ahead of it. This designates the telephone switch, just as the four-digit code identifies the customer. We call the two and then three digit code the prefix or exchange number. The prefix defines where a telephone is .
In 1947, the original 86 Numbering Plan Areas (NPAs) or area codes were laid out. Some of these area codes, like the 207 for Maine and the 302 for Delaware, are still in use today. As telephone subscribers grew the number of area codes have also grown. California is no longer restricted to two area codes but has more than 25.
The Bell System thought abbreviations would prevent misdialing, a mnemonic device to help callers unaccustomed to using dial telephones. To assist callers, AT&T’s William Blauvelt designed a dial with the letters and numbers we use today.