Enum is a method to provide a unified numbering system between the public switched telephone network (PSTN) and various VoIP providers. It is supported by some (but not all) service providers.
Technicaly speaking, Enum is a mapping of telephone numbers in E.164 format to domain names using a Domain Name System (DNS)-based architecture, specified in RFC3761. As official implementation using the e164.arpa domain is slow to progress, you can use Enum mechanisms with non-official but free registra like e164.org owned by enthusiastic people willing to test the Enum system (the Internet Telephony Users Association, a non-profit association). View the Enum implementations around the world
How does it work ?
ENUM works by acting as a look up service. A recipient registers their PSTN number with e164.org (or other ENUM registra) where they can provide a VoIP number, email address and URL.
- Provider registered number
Some providers automatically register the numbers that they provide to their users, in which case you don't need to do anything - it will just work.
- Registering your own PSTN number
You can register an account at e164.org's public ENUM with your PSTN number in international format (ie. +33 123 45 67 89). You can now add your email, your instant messaging account and your (Ekiga.net) VoIP number to this account.
Most registra require some form of authentication to prevent prevent people registering PSTN numbers which aren't theirs. e164.org does this by placing a call to the PSTN in question so that they can confirm that the real owner is the one attempting to register.
- Calling an ENUM
When a caller places a call to a registered phone number, the dialtone provider (PSTN or VoIP) does an ENUM lookup to find out whether the call can be placed via VoIP or PSTN, and then places the call using the cheapest route.
If a dialtone provider does not use ENUM, it is often possible to use a peering code (VoIP) or an access number (PSTN) to link to one which does.
Calling from PSTN network
- Using a phone provider that has ENUM lookup
The caller just dials the telephone number. Their provider will check automatically if this phone number exists in the ENUM directories and decides whether to route the call via VoIP.
If the phone number can't be found in the ENUM directories, the provider will use the PSTN to place the call.
- Using a phone provider that has no ENUM lookup
The caller will need to call via a phone service that links to an ENUM look up service. SIP Broker offers (via sponsors) access phone numbers to connect to its service from the PSTN. If the phone number doesn't exist within the ENUM directories the call is terminated.
For example, in France you may call 01-72-09-04-04 to reach SIPBroker and then dial *673 500# to call the ekiga's echo test.
Calling from a VoIP Network
Some VoIP service providers automatically do ENUM lookup on all numbers dialled, in which case the call will be routed via the best/cheapest route.
Unfortunately Ekiga.net has no automatic ENUM lookup, the user can use another network to do the ENUM lookup. This can be done via SipBroker, to place an ENUM call add *013 (the Sip Broker/ENUM server prefix) to the start of phone number.
Calling from Ekiga softphone
- If you call a SIP URL without a region (i.e. without a @) Ekiga will try an ENUM lookup. You have to start the phone number with the country code, i.e. 49... for Germany, 32... for Belgium, etc. So you can enter sip:43780004711 to reach the Austrian ENUM test number.
- The ENUM trees which are queried are hard-coded in the source code file ptclib/enum.cxx. They are:
e164.voxgratia.net e164.org and e164.arpa
Note that e164.arpa is the so-called "golden tree", i.e. the official ENUM tree.
- The ENUM lookup does not work in the version 2.0.11 for Windows, please use ekiga version 3.x.