E.164 – The modern dial plan

When we are addressing Voice over IP we need to remember that essentially we would like to reach customers over the PSTN or SS7 network. The only avenue to date to do this, is by using something called the telephone number. However, that number has undergone some changes recently.

The International Telecommunication Union (ITU) is the responsible body for all telephony numbering around the world. The way the numbering plan laid out is:

  • A telephone number can have a maximum of 15 digits
  • The first part of the telephone number is the country code (one to three digits)
  • The second part is the national destination code (NDC)
  • The last part is the subscriber number (SN)
  • The NDC and SN together are collectively called the national (significant) number

Now, each geographic area has responsibility for it’s own numbering plan. The United States and Canada share equally in the North America Numbering Plan (NANP) which entails a single-digit country code, followed by a 3-digit area code, a 3-digit prefix, and a 4-digit subscriber code. In other words, our numbering system is quite fixed. Other countries have variable length numbering plans, like England where the number grows based upon the density of a given city.

The E.164 has developed into something broader called ENUM (TElephone NUmber Mapping) which was the brain child of the IETF (internet engineering task force). Using the international E.164 number as a model, ENUM will assign a specific Uniform Resource Identifier (URI) to each and every networked device — including analog telephones and fax machines, smart phones, or computers. The reason for this is to make it easier to look up the numbers or devices using DNS servers on the internet.

The URI could be in a format of an email address (joe.parlas@cisco.com) or a number with an assigned domain name (+6197256999@cisco.com).  With URI all these devices will be able to contact each other directly using a single network address or phone number. ENUM also deals with storage of these numbers on the DNS (domain name server) so that voice over IP phones could look up a number over the internet and be connected to another voice over IP system.

RFC 3761 is defines the format of the number as:

The Application Unique String is a fully qualified E.164 number minus any non-digit characters except for the “+” character which appears at the beginning of the number. The “+” is kept to provide a well understood anchor for the AUS in order to distinguish it from other telephone numbers that are not part of the E.164 namespace.

For example, the E.164 number could start out as “+44-116-496-0348”. To ensure that no syntactic sugar is allowed into the AUS, all nondigits except for “+” are removed, yielding “+441164960348”.

So the full ENUM number must begin with a leading “+”. But the question remains, what does the “+” represent? Basically it represents your country’s access code to dial out to make international calls. For instance, if I were to dial an international number from my home in North America, I would begin by dialing “011” which is our code fore requesting international service.

In essence, a fully qualified ENUM number is one that could be dialed by any device, any place in the world, and the call would be properly set up. If you look closely at your cell phone when you dial from outside your home country you will notice that the cell provider has translated the number into a fully-qualified ENUM during the calling operation.

The beauty in this system is that I don’t have to worry about remembering international access codes for different countries, as long as the provider understands the leading “+” symbol.

Author: Joe Parlas

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