<P> As demand for telephone service grew in the post -- World War II period , it was foreseeable that the demand would exceed the addressing capacity of the existing system of using memorable telephone central office names as prefixes in telephone numbers . Several letter combinations had no pronounceable or memorable names and could not be used . Several North American numbering plan areas ( NPAs ) were divided so that more office codes became available to smaller regions . However , as the growth accelerated , the Bell System decided to implement all - number calling ( ANC ) and to deprecate the use of central office names to provide more central office codes to each NPA . This extended the usable numbering plan and only two area code splits became necessary between 1962 and 1981 . All - number calling was phased in starting in 1958 and most areas had adopted it fully by the mid-1960s . In some areas it did not become universal until the 1980s . The Bell System published and distributed area code handbooks yearly which compiled the towns available for calling using an area code . </P>
<P> As demand for telephone service grew in the post -- World War II period , it was foreseeable that the demand would exceed the addressing capacity of the existing system of using memorable telephone central office names as prefixes in telephone numbers . Several letter combinations had no pronounceable or memorable names and could not be used . Several North American numbering plan areas ( NPAs ) were divided so that more office codes became available to smaller regions . However , as the growth accelerated , the Bell System decided to implement all - number calling ( ANC ) and to deprecate the use of central office names to provide more central office codes to each NPA . This extended the usable numbering plan and only two area code splits became necessary between 1962 and 1981 . All - number calling was phased in starting in 1958 and most areas had adopted it fully by the mid-1960s . In some areas it did not become universal until the 1980s . The Bell System published and distributed area code handbooks yearly which compiled the towns available for calling using an area code . </P>
when did the phone company stop using letters
when did the phone company stop using letters
mid-1960s
mid-1960s