NANP telephone numbers consist of ten digits, of which the leading three are the area code.
As the number of subscribers grew, multiple exchanges served individual neighborhoods of large cities.
Multiple exchanges were identified by a central office name and typically four digits, such as "Pennsylvania 5000".
Various methods were used to convert these to dialable numbers as dial systems replaced manual switchboards; many moderately-large cities used a 2L-4N format where "ADelaide 1234" would be dialled as AD-1234 (23-1234, a six-digit local call).
In some small villages with only one local exchange, it may have been permissible to dial only the four-digit station number.
Even after exchange names were introduced, it was possible in many small cities to call local numbers by dialing only the five digits which followed.
It was also eliminated as early as 1981 in some numbering plan areas in the United States that had introduced interchangeable central office codes.
In Operator Toll Dialing, automated equipment translated area codes to routing information.
Systematic conversion of city dial systems commenced to the seven-digit (two-letter-five-number) numbering plan.
This decade is notable for implementation of some thirty more area codes, including in Alaska, Hawaii, and the Caribbean.
Service demands in the largest American cities of New York and greater Los Angeles area resulted in the first use of interchangeable central office codes (NXX).
As 1994 neared its end, ten-digit dialing became required throughout the numbering plan in preparation for interchangeable NPA codes.
The concept of a ten-digit local number was conceived, as New York had an overlay code (917, implemented in 1992), but seven-digit dialing was still the norm.
Although fewer American area codes were overlaid, seven-digit dialing was also disappearing in the United States.
On July 16, 2020, the FCC adopted rules to establish 988 as the three-digit phone number for the National Suicide Prevention Lifeline.