 This increased the number of effectively available central office codes in each numbering plan area (NPA) from 540 to 792, thereby staving off the threat of exhausting the number pool, which was forecast to occur by the late 20th century.Until the 1950s, a typical telephone number in the United States and many other countries consisted of a telephone exchange name and a four- or five-digit subscriber number.The first two or three letters of the exchange name translated into digits given by a mapping typically displayed on the telephone's rotary dial by grouping the letters around the associated digit.