<Tr> <Td> 99x </Td> <Td> WYandotte, WYman, WYndown </Td> </Tr> <P> Fictitious phone numbers starting with 55 used the fictitious exchange name KLondike (55). The letter Q was never used in the naming system, but Z was often mapped on the telephone dial to the digit 0 (zero) to facilitate dialing of Zenith numbers . </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> <P> Experiencing significant resistance in many areas, the Bell System employed a strategy of gradual changes to ease the transition for customers . Originally, directory listings were printed with the central office name spelled out in full, e.g., </P>

When did the phone company stop using letters