- The code, or prefix, which identifies a central office. In the NPA-NXX-XXXX ten-digit telephone number format, it is the NXX portion, where N can be any digit between 2 and 9, and X can be any digit between 0 and 9. A single central office may serve more than one NXX code. Also called central office prefix, central office code, or prefix (1).
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