National and international numbering plans

A numbering plan is a scheme for allocating unique numbers to subscribers, exchanges, areas and countries, so that every telephone in the world can be reached and the switches can route a call from the dialled digits. It also fixes the number length, prefixes and special codes.

Requirements of a good numbering plan

• A unique number for every subscriber.
• Easy for users to remember and dial.
• Allows routing and charging from the digits.
• Room for future growth without renumbering.
• Follows ITU-T recommendation E.164.

Types of numbering plan

1. Open numbering plan. Number length varies; a trunk prefix is dialled for calls outside the local area (e.g. 0 + area code). Used in Nepal, the UK and India.
2. Closed numbering plan. All numbers in the country have the same length and the full number is always dialled; there is no trunk prefix. Examples: USA/Canada (NANP, 10 digits), Singapore (8 digits).
3. Uniform numbering: all subscribers have the same number length.
4. Non-uniform numbering: different areas use different subscriber number lengths.

National numbering plan

A national number consists of

[Trunk prefix] [Area/trunk code] [Exchange code] [Line no.]
      0              1               44            12345

• Trunk prefix (0): tells the exchange that a long-distance call follows.
• Area (trunk) code: identifies the numbering area.
• Subscriber number: exchange code + line number.
• National significant number (NSN) = area code + subscriber number.

Examples in Nepal: Kathmandu 01-4XXXXXX, Pokhara 061-5XXXXX, Biratnagar 021-XXXXXX; mobile numbers have 10 digits, e.g. 984XXXXXXX (Nepal Telecom), 980XXXXXXX (Ncell); special codes 100 (police), 101 (fire), 102 (ambulance).

International numbering plan (ITU-T E.164)

[International prefix] [CC] [National significant number]
         00            977        1 4412345

• International prefix: dialled to leave the country (00 in Nepal and most countries, 011 in North America; written as "+").
• Country code (CC): 1 to 3 digits; the first digit gives the world zone.
• NSN: area code + subscriber number, without the trunk prefix.
• The maximum length of the international number (CC + NSN) is 15 digits.

World numbering zones:

Example: a Kathmandu number 01-4412345 called from abroad is dialled as 00-977-1-4412345 (written +977 1 4412345); from Pokhara it is dialled 01-4412345; from within Kathmandu, just 4412345.