This varied by city and phone company, but for large cities whose phone companies were part of the Bell System, it was usually after World War II. In 1948, for example, Chicago shifted to 2 letter + 5 digit dialing, already in use in some other big cities and becoming a Bell System standard. The national standardization of numbers wasn’t needed until subscriber long-distance dialing was introduced in the 1950s and 1960s. Localities with four-, five-, or six-digit numbers were common into the 1960s, and some rural pockets didn’t get connected to the national network until the 1980s.

Dialing a phone number yourself was introduced in big cities in the 1920s. Before that, you asked to be connected to a distant building where phone lines converged, called an "exchange," then the number of the line within that exchange. The letters on a phone dial let you dial the first three letters of the exchange name (which were numerically unique), and then the four digits of the particular line. Subscriber dialing required new central office equipment, and different parts of a large city might get the new system at different times. In the early days of telephony, big cities had competing telephone companies, especially in business districts, so ads often list two telephone numbers, such as Central 415 and Automatic 2266. In 1900, automatic exchanges were still rare, so someone at the Knick would have to tell an operator the number or name of the person he wished to reach.

In the 1940s, the pattern was changed in most cities so you only dialed two letters to indicate the exchange, then five numbers for the line. There were always useability drawbacks to the named exchange system, and a couple of combinations (like 95) didn't spell anything, so all-number dialing was the standard for new phone numbers by the 1970s. Since phone book publishers only reset entries that needed to be changed, phone books still listed old-style numbers until the early 1980s.