ENUM was developed as a solution to the question of how network elements can find services on the Internet using only a telephone number, and how telephones, which have an input mechanism limited to twelve keys on a keypad, can be used to access Internet services. ENUM at its most basic is the convergence of PSTN and IP networks; it is the mapping of a telephone number from the public switched telephone network to Internet functionalities.


ENUM enables Internet-based users to make a selection from a range of services available for communicating with another person when the caller knows only a telephone number or has access only to a telephone keypad. ENUM allows users to access Internet-based services and resources from Internet-aware telephones, ordinary telephones connected to Internet gateways or proxy services, and other Internet-connected devices where input is limited to numeric digits.

The principal applications for ENUM have centered on two areas. One is Voice over IP (VoIP) and the other centers on Voice Protocol for Internet Mail (VPIM). The long-stated goal of the VoIP industry has been to make a phone call over the Internet as easy to make and as high-quality as a regular PSTN phone call. The goal of the VPIM industry has been to develop a comprehensive mechanism whereby voice mail systems could exchange messages over IP networks. ENUM enables carrier and enterprise voice mail systems to find each other, interoperate, and exchange messages.


If an average telephone customer was to make a telephone call using, for instance, an Internet-enabled phone to another Internet-enabled phone, all of the steps in between are invisible to the user. To the call initiator and call receiver, this phone call would appear the same as a call made over the PSTN. Subscribers will, of course, have to identify the ENUM services that they wish to use.


How does ENUM work?

Once a telephone number is entered, it is translated into an Internet address using the following steps:

  1. The phone number is translated into a fully qualified E.164 number by adding the city (or area) and country code. Example: 555-1234 dialed in Washington, DC becomes %201-202-555-1234, where the "1" represents the North American country code. The "%20" indicates that the number is a fully qualified E.164 number.
  2. All characters are removed except for the digits. Example: 12025551234
  3. The order of the digits is reversed. Example: 43215552021
  4. Dots are placed between each digit. Example:
  5. The domain "e164.arpa" is appended to the end. Example:

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