System information

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