Enum can be thought of as a DNS for Telephone Numbers. For the unitiated, DNS lets your PC work out how to get to www.westlake.co.uk on the internet. For example, your web browser would look up www.westlake.co.uk and DNS would return 77.92.65.28, which is the IP address for the server that the webiste is hosted on. So Enum does something very similar.
Give Enum a telephone number and it will provide you with an address, such as an IP address, to send the call to. In theory this would allow your PBX to send calls directly to a third party's PBX without even previously having connected to them, using SIP. Unfortunately, Enum is actually quite difficult to understand and there are a number of reasons you need to think through your use of Enum.
Enum is actually a means of mapping a telephone number (E.164 to be precise, +44 etc) to the internet addressing system. Here is an example:-
1. I want to call +4401708300013
2. My PBX requests Enum Details for +441708300013
3. Enum returns a series of complicated addresses for all the methods that this number can be contacted by. Methods could be SIP, email, IM or even PSTN.
4. In this example, I want to make a voice call, so the PBX slects the SIP address: sip:441708300013@sip.ipsip.co.uk
5. The PBX can now make the call via SIP directly to the IPSIP SIP Proxy.
Enum has more uses than just for voice. If I wanted to send an e-mail but I only have the person's phone number then I could type the number into Outlook and Enum could be used to look up the email address for me. Just think of Enum as a telephone number that will know all the ways you can be contacted. The Client (voice, email, Instant messenger) decides which connection method to use.
