# Restriction Class of Service - Overview

The Restriction Class of Service (RCOS) acts to restrict system outdials. Restrictions can be programmed by number of digits to dial (for example, a limit of 7 digits effectively restricts long-distance calling), by screening for certain area codes and Central Office codes, using NPA/NXX screening.

## NPA/NXX Screening

NPA (or Numbering Plan Area) is the 3 digit code in a telephone number that represents the area. (For example, one area code for Ottawa is 613.) The NXX digits represent the Central Office or exchange that is assigned to the CO that serves this telephone number. (For example, the NXX digits for the CO that serves Mitel are 592.)