18.1.1 How an ENUM Trunk Works
When a call is made via an ENUM trunk, the system will start by accessing the ENUM registry and attempting to convert the number dialed into a SIP-URI. If the system manages to do this, a standard call will then be made directly via SIP to the opposite terminal with this SIP-URI.
The goal of an ENUM is to make different addresses, numbers and URLs available under a single number. This makes it possible for the user to be reached under the same number in the Internet and in the classical telephone network.
SwyxWare supports the DNS addresses *.e164.arpa (official ITU project) and the alternative project *.e164.org for the ENUM number resolution.
An ENUM trunk directly accepts the SIP messages from the opposite terminal. They are not forwarded via a SIP provider (as is the case with a SIP trunk).
Fig. 18-1: SIP without Internet service provider (ITSP)
To call a SIP client (this can be a terminal device or a software-based telephony system) via an ENUM trunk, the corresponding SIPURI (Uniform Resource Identifier) of the client must be known. If you call a SIP client, for which there is a SIP URI, as usual via the telephone number, the corresponding SIP URI will be queried with the help of the ENUM application and used for creating a connection.
This means that ENUM uses in the case of a call the telephone number dialled (e. g. +1 202 555 1234), transforms it into a domain name (e. g. 4.3.2.1.5.5.5.2.0.2.1.e164.arpa.) and makes a DNS request. As part of this process, records known as NAPTRs (Naming Authority Pointer Records) are sent back. The evaluation of these records results in one or more different types of URI (e.g., SIP:name@domain.com or mailto:name@domain.com, etc.) under which the desired service of the given domain can be contacted.
SwyxWare currently supports the SIP URI type. The use of additional URI types (e.g., mailto, http, etc.) will feature in a later SwyxWare version.
If a SIP URI exists, communication with the SIP client can take place.
Fig. 18-2: ENUM call process
Please note that communication via an ENUM trunk can only take place with opposite terminals that are able to ensure the existence of a SIP-URI. If the DNS server cannot provide a SIP-URI for the number dialed, it will not be possible to establish a connection.