 The telephone number was further distinguished as consisting of three digits that identified each central office and four digits for each subscriber station connected to the same central office.From 1947 to c. 1960, the initial two digits of the central office code were mapped to two letters of an often locally significant name, the central office name or exchange name.However, this scheme, practiced since its invention by W. G. Blauvelt of AT&T in 1917, prevented the use of numerous digit combinations that could only be associated with unpronounceable or, at best, confusable names.