The trailing digits of a telephone number. As defined by the North American Numbering Plan (NANP), the line number can consist of any four digits (XXXX), and corresponds with a port on a switch that connects to a circuit or channel over a local loop that serves the physical premises of a subscriber. In the case of a cellular telephone number, there is no local loop, as such, and the number is associated with a handset, rather than a physical premises. See also
NANP.