The sub-NPA code in a TN, i.e., digits D-E-F of a 10-digit NANP Area address. Central office codes are in the form "NXX," where N is a number from 2 to 9 and X is a number from 0 to 9. Central office codes may also be referred to as "NXX codes. " (47 C. F. R. § 52. 7 (c) )