Line Number
NANP · also called subscriber number, station number
The last four digits of a phone number — the subscriber number within a central-office code.
The line number (or subscriber number) is the final four-digit block of a ten-digit NANP number: the XXXX in NPA-NXX-XXXX. It identifies an individual line within a central-office code, giving up to 10,000 numbers per NXX.
Under number pooling, the first of those four digits also marks which thousands-block the number belongs to.
Common misconception.
The line number alone is not unique — it only identifies a line within one NXX in one area code. The full ten digits are what make a number unique.