Telephony Phone Numbers

In much the same way that an IP address is used to identify and route traffic to a unique host on a TCP/IP network, phone numbers are used to identify and route traffic to a particular destination phone line. In the world of telephony, these numbers are designated and defined by a combination of international and national organizations. For example, the International Telecommunication Union Telecommunication Standardization Sector (ITU) defines the international numbering system, such as the country codes used in conjunction with a long distance call. Within a country, various governing bodies define a numbering plan based on factors like defined areas, and projected number of lines required, As outlined in the main ITU E.164 specification. In North America, telecommunications providers use a system known as the North American Numbering Plan (NANP). In NANP, phone numbers are 10 digits in length, according to the properties shown below:

3-digit area code. The first 3 digits in a 10-digit phone number identify an area or geographic region. For example, a particular city will often have one or more area codes associated with it. For the sake of clarity, the area code associated with a phone number is usually displayed in parenthesis, for example (416). In many regions, the area code only needs to be dialed if attempting to reach a different area, in which case the digit “1” usually precedes the number. However, in many areas, especially those with multiple area codes associated with a single “local” calling area, all ten digit must be keyed, usually without the preceding “1”, which is used to identify a long distance call.

3-digit CO code. The next 3 digits in a 10-digit phone number usually identify a particular CO switch. As the user dials these numbers, the call is forwarded to the CO switch to which the user line is connected. For example, the CO code “555” is likely associated with a particular switch. When the user keys in “555” on their handset, the call is forwarded through the various CO switches to the switch associated with that CO code. Of course, one CO switch is usually responsible for multiple CO codes. If you consider the 3-digit CO codes used by your phone number and those of close neighbors, this should become clear.

4-digit line code. The last 4 digits in a 10-digit phone number identify a specific phone line connected to the CO switch. For example, in the phone number 555-8936, the “8936” portion identifies the specific line number to which the call should be forwarded on the “555” switch. After the “8936” portion is keyed in by the user, the “555” CO switch sends a ringing tone on the “8936” line.

The numbering plan in use in your home country likely differs from the 10-digit plan specified for North America. However, most countries use an internal numbering system that follows the recommendations outlined in the ITU E.164 specification.

Author: Dan DiNicolo

Dan DiNicolo is a freelance author, consultant, trainer, and the managing editor of He is the author of the CCNA Study Guide found on this site, as well as many books including the PC Magazine titles Windows XP Security Solutions and Windows Vista Security Solutions. Click here to contact Dan.