Details

Category: Technology

• VoIP

When you see a number, have you asked if it is a random number?

Short answer: no. There is logic behind it. That logic is written on the E.164. The E.164 is the standard that dictates the nomenclature of the telephone numbers. In this article, I will try to explain the logic of that document easily.

The Structure of the International ITU-T E.164 Number

The first thing to understand is that the numbers are classified into geographical, global services, network, groups, trials. Geographical numbers are the ones that are tied to a specific geographical area (although we already know you can have a New York City number if you live in Los Angeles, but let's bear with this).

Geographical Numbers

The geographical number is composed of decimal digits that form the country code (CC) and the national significant number (NSN). Each country will divide their NSN according to its needs. There are some basic rules:

GN = CC + NDC + SN

• The total length of the number can not exceed 15 digits.
• The Country Code (CC) length is between 1 and 3 digits.
• The National Destination Code (NDC) length depends on the Country Code:
  • four digits if the CC has three digits,
  • five digits if the CC has two digits, or
  • six digits if the CC has one digit.

International Global Services Numbers

International global services number follows these rules:

IGSN = CC + GSN

• The total length of the number can not exceed 15 digits.
• The Country Code (CC) length is three digits.
• The Global Subscriber Number (GSN) can not exceed 12 digits.

International Network Numbers

This kind of number is composed of three code fields as follows:

NN = CC + IC + SN

• The total length of the number can not exceed 15 digits.
• The Country Code (CC) length is three digits.
• The Identification Code (IC) length is from one to four digits.
• The Subscriber Number (SN) maximum length is 12. However, there is a minimum length that varies:
  • nine digits with a one-digit IC,
  • eight digits with a two-digit IC,
  • seven digits with a three-digit IC, and
  • six digits with a four-digit IC.
• The length of the IC and SN cannot exceed 12 digits altogether.

International Group Numbers

This kind of numbers is composed of three code fields:

IGN = CC + GIC + SN

• The total length of the number can not exceed 15 digits.
• The Country Code (CC) length is three digits.
• The Group Identification Code (GIC) length is one digit.
• The Subscriber Number (SN) maximum length is 11.
• The length of the GIC and SN cannot exceed 12 digits altogether.

Trial Numbers

Trail Numbers have two mandatory and one optional code field as follows:

TN = 991 + TIC + SN

• The total length of the number can not exceed 15 digits.
• The first prefix length is three digits, and it always seems to be 991.
• The Trial Identification Code (TIC) length is one digit.
• The Subscriber Number (SN) length is 11 digits maximum.
• The length of the TIC and SN cannot exceed 12 digits altogether.

Country Code Assignation

Canada doesn't get the +1 just for fun, Mexico +52 or Australia +61. There is logic within, and the best way to understand is through this graphic.

The + Sign

Commonly substituted by 00 in some PBXes, the + sign is a suggestion from the E.123 to indicate that an international prefix is required. This could make a difference, for example:

• 6138007370, depending on the context, could be an Ottawa, ON number or an Australian one.
• +6138007370, refers to an Australian number.

The Zero

The zero has a special meaning. Commonly used to identify Global or Service numbers. Depending on the Country, the zero can make a difference between a valid number and an invalid one. For example, +1100XXXYYYY is an invalid geographic number, but it could be a valid service or network number.

I will keep updating this article as I understand more about this subject.

Good luck!