ENUM and E.164

Although the SIP protocol really doesn’t think in terms of phone numbers, the reality

is that phone numbers are not going away any time soon, and if you want to properly

integrate a VoIP system with as many telephone networks as possible, you’re going to

need to handle the PSTN in some way.

ENUM maps telephone numbers onto the Domain Name System (DNS). In theory,

ENUM is a great idea. Why not cut out the PSTN altogether, and simply route phone

calls directly between endpoints using the same numbering plan? We’re not sure this

idea is ever going to become what the emerging telecom community would like it to

be, though. The reason? Nobody really can say who owns phone numbers.

E.164 and the ITU

The International Telecommunication Union (ITU) is a United Nations agency that is

actually older than the UN itself. It was founded in 1865 as the International Telegraph

Union. The ITU-T sector, known for many decades as CCITT (Comité consultatif

international téléphonique et télégraphique), is the standards body responsible for all

of the protocols used by the PSTN, as well as many that are used in VoIP. Prior to the

advent of VoIP, the workings of the ITU-T sector were of little interest to the average

person, and membership was generally limited to industries and institutions that had

a vested interest in telecommunications standards.

ITU standards tend to follow a letter-dot-number format. ITU-T standards you may

have heard of include H.323, H.264, G.711, G.729, and so forth.

E.164 is the ITU-T standard that defines the international numbering plan for the

PSTN. If you’ve ever used a telephone, you’ve used E.164 addressing.

Each country in the world has been assigned a country code,

‡

and control of addressing

in those countries is handled by the local authorities.

E.164 numbers are limited to 15 digits in length (excluding the prefix).

In Asterisk, there is nothing special that needs to be done in order to handle E.164

addressing, other than to make sure your dialplan is suitable to the needs of any PSTN-

compatible channels you may have.

For example, if you’re operating in a NANP country, you will probably need to have

the following pattern matches:

_NXXNXXXXXX

_1NXXNXXXXXX

_011X.

_N11

‡ With the exception of 24 countries and territories in country code 1, which are all part of the North American

Numbering Plan Authority (NANPA).

ENUM and E.164 | 247

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